I filed a bug report - just described what was happening. I didn't attach a project (it's not mine to share), but I linked to this thread, GRR! I just checked and the ticket I raised can't be found. WTAF?

Done the same. If you need to launch under Rosetta (due to 3rd party binary issues) use arch -x86_64 /Applications/Xcode12.app/Contents/MacOS/Xcode