What started this mentality?
Yes, absolutely, and it's among the worst ways to develop software. Designers kept coming up with controls that required coding custom behavior not already provided by the toolkit, either by modifying and extending components or creating new ones from scratch. Designers blamed developers for causing "rework" when it was found that the design included things that didn't exist, and developers took the blame when it took longer to implement the mocks than designers had led project leadership to believe. The lead designer's solution was to more-or-less abandon the team's pretense of being agile and get everything completely frozen well in advance of any coding.
Also, the backend was horrific, as might be expected when leadership treated it as an afterthought. I got the impression that because the system was built to replace an existing front end, leadership believed that it was just matter of wiring the UI up to the services that already existed. Some very legacy services, think mainframe-era fronted by a thin SOAP-to-json layer. Yeah and some of the backend services that turned out to be required didn't even exist.
I'm glad I'm done with that consulting gig. It was not fun, it wasn't challenging, it was just a grind, and if it is complete and they are able to turn off the existing front end on schedule I'll be shocked, and I'd want to know what kind of dumpster fire they end up with.