Scientists usually have a very poor understanding of the history of science, generally speaking. They do have an idea of their field's history in their head — but it's usually filled with myths and bad historical frameworks. (Much of it is what we call "whig history," or "a history as told exclusively from the perspective of the present, cast backwards in time.") This is not just because they are not professional historians, but because they usually do not read the output of professional historians (more typically, they read other scientists or pop historians on these subjects). They regurgitate the myths they know in their own classes and textbooks, and these get propagated to the next generation of scientists.

But here's the real rub: they prefer the myths and bad history to the good stuff. It's not just a matter of them having not been exposed to it (though they usually aren't). The mythic history of science is one that not only gives their career a narrative they are happy to be part of, but also gives them exemplars for what it means to be a good scientist. So this kind of bad history does "work" for them, and it can be quite useful within the field of science they work on, even if it's totally wrong (which it often is).

A brief example of this: scientists generally like to believe very simplistic versions about the Galileo affair, in which Galileo, the great scientist, is pitted against a mob of religious ignoramuses. This isn't how it really went down, and the real Galileo affair is notoriously complicated, and not nearly as clear cut a "science versus religion, truth versus dogma" sort of affair. But the mythical Galileo affair turns the history into a morality tale: it's about the sacrifices of scientists in the name of truth, it's about what happens when scientific authority is superseded by religious or political authority, and it's a "good versus evil" story grafted into a "science versus the Church" story. You don't get that simple "moral" when you look at the details of it, and most scientists are totally uninterested in the actual complexities of the time.

There is a large and robust discipline of the History of Science, with PhD granting institutions and professorships and all of that. (My PhD is in the History of Science.) It is a field of history and requires as much work as any other sort of historical study. In some cases it requires more work than other fields of history, because it also often requires an understanding (to some level of sophistication) of the technical concepts under study, including historical technical concepts that nobody contemporary looks at anymore (e.g., aether theory or phlogiston). It's a real thing, and we have conferences and journals and so on.

Some scientists actually do endeavor to do good historical work, but they are rare compared to the total number of scientists out there, and requires a lot of dedication and serious study (because history is an entirely different field of expertise). But many historical and practicing historians of science were once scientists to one degree or another.

In short: if you want to know about the history of science in a serious way, look to the historians of science. If you are instead looking to history as simply a way to digest present-day scientific concepts and do not really care it if is accurate, you can look to scientists and science-writers.

It probably depends on what you want to know about that field. Let's take music: do you want to know the technical stuff about music in history? E.g., a deep dive into the way that the Beatles used harmony, or how a sonata form developed? In that case, you're probably going to be looking at things written by musicologists/music theorists. Or do you want to know about how that particular form of music developed in a particular culture, responding to political or social developments? In that case, you're probably going to be best off reading something by a historian.

Put it this way: the skills of a professional historian are the skills of being able to evaluate information. A professional historian is very skilled at sifting through information, tracking down archives, being able to weigh biases in accounts, and of being familiar with the historiography of a period (e.g., the debates in a field about how best to interpret the information we have). Historians are very good at poking holes in previously existing narratives about what happened during a time period, and reinterpreting what that might mean. All of this is going to be helpful in a bunch of ways to interpret the history of a particular field - whether music or science.

Typically speaking, your average person who is very skilled at analysing the music doesn't always know this historian kind of stuff that well - you can analyse the way that Beethoven uses plagal cadences in his music until the cows come home without needing to be able to read German in old-style gothic script. But without understanding German in old-style gothic script, and without knowing what a historian would know about Beethoven's milieu - what it was actually like to live in Vienna around the turn of the 19th century - the person who can otherwise analyse those cadences would be quite limited in contributing to our understanding of Beethoven as a person, what motivated him, what things he came up against, etc.

With the Beatles, for example, there's a bunch of mythmaking going on - the band are still a big commercial concern and those commercial concerns have long influenced how they tell their histories (and still do). Ringo's and Paul's memories of the 1960s are probably pretty foggy by now. The baby boomer generation sees the Beatles as symbolic in a bunch of ways. Their experience of the Beatles at the time is certainly part of their story, but the prominence of the baby boomer perspective in popular rock history can certainly obscure what the band actually were, and can lead to a very 'Whiggish history' view of the music which sees everything that led to their music as primarily interesting because it lead to their music rather than being interesting in of itself for what it was trying to do; you can see this Whiggish history perspective to some extent in a lot of the obituaries of Little Richard recently, for instance. A historian would simply be much better equipped than a musician to cut through the mythmaking to get at a more realistic perspective of what actually happened.

From my perspective, Walter Everett's very detailed musical analysis at the music of the Beatles in the two volumes of The Beatles As Musicians is very perceptive about the Beatles' music and what it is aiming to do, but then so is Mark Lewisohn's Tune In, the first part in a three part biography/history of the Beatles. Lewisohn is the one scouring over archives of Liverpool newspapers in the 1930s; Walter Everett is the one pointing out the consistencies in the way that the Beatles explore drones on the album Revolver.

Mind you, my experience is that Walter Everett knows the intricacies of the Beatles' history pretty well, and Mark Lewisohn's writing suggests he has at least a passing familiarity with music and how it works, even if it's not his focus in the same way it is for Everett. Many people whose research is interdisciplinary (as music history by definition is) do of course have significant skills and knowledge in both fields - for example, /u/restricteddata who studies the history of nuclear science has a flair both here at /r/AskHistorians (whether they answer questions about the history of science more generally) and at /r/AskScience (where they explain things about nuclear science more generally).