unfortunately this is a rather vague report, as it's not 100% reproducible. nevertheless i wanted to report it:
sometimes (conditions are still not totally clear to me), the clangd code model for specific sources is not updated after a file changes. this tends to happen more often when the source file is changed externally (e.g. via git rebase). even modifying the file won't update the code model (but the clang-tidy hints are refreshed).

the most notable effect is that clangd parse errors are listed among the annotations and that code navigation stops working. the only way to get out of this situation is to restart qtcreator.

interestingly though: the c++ code model inspector for the file seems to be up to date, most notably, the "diagnostic messages" do not match the ones that are shown as annotations in the editor