Can a department revoke a degree, once granted by the university?
Absent plagiarism, fraud, or other academic sanction, it would be difficult to revoke an awarded degree. There would have to be strong evidence for doing so, and it’s not often attempted, in part because of the likelihood of legal action.

So, I would normally say don’t worry about it. However, in a dysfunctional department, anything could happen, and you should be prepared to act in that unlikely event.