From what I've read about the Roman empire, the shields that soldiers used before the Marian reforms were primarily circular, much like the hoplites of the times.
Then after the reforms, the rectangular scutum became standardised and quite a few documentaries made a point of talking about how effective they were in combat, to the point of insinuating they were the cornerstone of Roman expansion.
But in all the pictures I've seen and what I've read about the late imperial era, the rectangular scutum seems to have disappeared again, replaced by ovular shields.
If the documentaries I've watched are right (it's been a while, I don't remember their names sorry) and the rectangular scutum really was overpowered (for lack of a better term), why was it phased out? Was it phased out?
Not sure what documentaries you watched but I wouldn't pin Roman success on their shield. Their success is more likely due to a massive supply of manpower, political opportunism and aggressiveness, mastery of logistics, and an effective army.
Firstly they hoplite aspis was replaced after the Samnite wars when the Romans switched from the hoplite model to the manipular system that they used (with some refinements such as the switch to cohorts) throughout the rest of the Republic and the Empire. The new scutum was originally oval and very tall (on mobile so can't link an example) and used throughout the Republic. Under the Principate they moved to the square scutum, not sure if we have a reason but I'd guess to reduce the weight.
Again not sure of the reason for the switch to a flat oval shield, if I were to hazard a guess it would be due to the changing tactical usage of the legion.