That e.g. a form should work predictably according to some unambiguous set of principles is of course a UX concern. If it doesn't, then maybe someone responsible for UX should be more involved in the change review process so that they can actually execute on their responsibility and make sure that user experience concerns are being addressed.

But sure, the current state of brokenness is a result of a combination of overambitious designs and poor programming. When I worked as a web developer I was often tasked with making elements behave in some bespoke way that was contrary to the default browser behavior. This is not only surprising to the user, but makes the implementation error prone.

One example is making a form autosubmit or jump to a different field once a text field has reached a certain length, or dividing a pin/validation code entry fields into multiple text fields, one for each character. This is stupidity at the UX level which causes bugs downstream because the default operation implemented by the browser isn't designed to be idiotic. Then you have to go out of your way to make it stupid enough for the design spec, and some sizeable subset of webpages that do this will predictably end up with bugs related to copying and pasting or autofilling.