frontend expects the backend to return errors, but the backend lies and says everything was successful, and then certain things break in the UI
That’s a double failure then: not only does the backend do it wrong, the frontend devs don’t even know it. If they’d agreed on one way of handling it, they’d still be able to work it out. But if the devs don’t even communicate their standards with each other and the frontend devs obviously don’t know about the problem…
How is this getting even worse still? Why would a single endpoint work differently? That suggests they have standards, just one arsehole decided to shit on it and the rest didn’t vet their code enough to catch the issue.
I worked QA in a small dev team for two years. You might be surprised how uncomfortable you can make some developers within five minutes of “basic testing”. By the time I left, the team lead loved me as much as some of the devs must have hated me.