Upd: The problem came back, and I managed to find the fix: it is to do with "live issues" in xcode settings->General!! Uncheck it and everything works fine. https://stackoverflow.com/questions/73808435/xcode-errors-disappear

Thank you! This fixed the problem for me also! I'd encountered this problem in the past, and have fixed it a different way last time, but couldn't find that other thread/forum... It was something in the settings, to do with live error showing or cloud stuff, can't remember for the life of me =( But this solution also worked!