If the compiler from Xcode.app directly is set up for the kit (instead of /usr/bin/...), Clangd misses the sysroot setting and shows errors. So far that is probably expected.

After changing the compiler in the kit, the errors persist. Restarting Clangd doesn't fix it either. I guess the reason is that the compiler_commands.json is not updated. I also do not find a way to trigger that manually.

Things go back to normal after restarting Qt Creator or switching sessions, or closing/reopening the project.