This is probably from unexperienced users who put these brackets (or something was removed from fraction and the brackets were no longer necessary), but I guess there are also constructs where brackets give it a different meaning (Legendre symbols is one that comes to mind)

Unfortunately I guess in many cases there is no precise way to distiguish meaning from notation only...

yea, i can definitely try to exhaust cases where I am certain there is little ambiguity to canonicalize things, but it takes time and effort, most importantly, it is not what can learn a lot from doing these tiny fixes. however, I am interested to investigate some machine learning methods that can canonicalize things more automatically, if I can get a chance to do it in my future research, definitely will do.