The thing is the term "Medieval Japan" covers a really long time where many social changes took place and where the strength of a relation between samurai and their lords wasn't always the same. Not only that, the very word "samurai" did not have the same meaning during the early Medieval period, around the 12th century, than in the 17th century, once the Tokugawa shogunate had been established. The word originally meant servant, and those who were called by it were mostly foot soldiers.

It's true than in the Edo period (17th-19th century) there was a "revival" of Bushido, since there weren't that many wars to occupy the idle samurai and they begin to question what is their value and their purpose as a social class. It is then when the idea we have nowadays of what samurai are is formed. Hagakure, also called The Book of the Samurai wasn't written until 1715 or so. When actual war was going on in the country, courtesy and honour weren't as relevant as victory in many occasions. Oda Nobunaga was forced to commit suicide by one of his generals; Tokugawa Ieyasu didn't let his pledge to Toyotomi Hideyoshi prevent him from taking the country from his liege's son, etc.

That said, there are examples of strong honour bonds between lords and retainers before Edo. In the Heike Monogatari, which talks about the 12th century Genpei wars, we find many cases. There's for instance the story of Taira no Tadamori's retainer, who guarded his master's door in the palace, threatening the noblemen who wanted to kill him with his sheathed sword. It was illegal to carry weapons in the palace, so when they asked Kiyomori to respond for the man's insubordination, the retainer showed that it was actually made of wood; so the law hadn't been broken.

Edit: a word.

Edit 2: It wasn't Taira no Kiyomori's retainer. It was his father's.

During times of warfare, bushido was not a universally recognized code that everyone had agreed on. It's what we call the various customs and social agreements made between a vassal and his lord. These 'laws' or 'codes' (they weren't codified until the Edo Period, long after those wars were over) were only meant for warriors and the warrior class.

They never governed the life of the vast majority of Japanese people.

And these codes were violated all the time. For one thing, there was no institution of justice that was going to judge who followed the the customs and rules and who didn't. In a period of constant warfare, intrigue, and king making, samurai were practical and opportunistic.

Many samurai would betray their masters if it meant moving up in the world, and we see it happen time and time again. And betraying one's master or disobeying one's master was basically the antithesis of bushido.

It was common for 'reinforcements' to arrive at the scene of a pitched battle and then sit there and wait to see which side was more likely to win. They might even negotiate. Many times, allies that came to reinforce your side might suddenly become the enemy.

So no, modern pop culture often overplays the idea of samurai bound by honor and tradition. Samurai were more than willing to violate honor, tradition, agreements, treaties, and social norms if it meant winning. This included assassinations, holding family members hostage, killing said family members, blackmail, betrayal, selling information, switching sides, etc.

This doesn't mean that tradition and honor weren't large parts of their lives, because they were. But at the end of the day, tradition will not stand in the way of the ambitions of a aspiring warlord.

I apologize for not having the time to write out a direct reply to your question. The long and short of it is that Bushido was made up in the 19th century. I've written about bushido here which might help answer your question. Another related topic I've written on is warriors switching sides during battles. It's quite short, but hopefully illustrative. The link is here.