I compiled yesterday's MAME 0.182 and today's QMC2 0.71 without issues - and the BGFX_metal bug still persists.
But I finally made it into mametesters.org and managed to file a bug report here. As it turns out today, it was the MacOS Password Manager and Safari that kicked me out from mametesters.org at previous attempts. A manual login via Firefox after password reset did the trick. So if you want to login to Mametesters as a Mac user, I strongly recommend you to avoid Safari and Mac Password Manager if you run into strange problems like me, being blocked from the site. If you are at Mametesters.org and are able to confirm the bug, please leave a note there - thanks!
Update: While the Mametesters Bug Report has been closed, a Bug has been opened with the BGFX Github repository: https://github.com/bkaradzic/bgfx/issues/1032 - let's see.