we had a lot of very talented iOS devs that started running away with feature development when the server team couldn't keep up.

This really hurt the android + web client teams.

Eventually our backend started changing (mono-rail -> microservices) and it turned into an absolute cluster of trying to massage/cram new data models into the existing ones the iOS team created.

Late stage startup and then post finding product market fit problems.

OP is likely talking about local business logic, ie password field is min 3 chars long. You validate that in the FE before sending it up to get instant feedback to the user (yes you also have it on the server).