No, the bug is not reproducible all the time. I have switched to Emacs 28 since then, as this breaks functionality important for me. Now tried to run 29 again, but it doesn't happen anymore, neither with `-Q' nor without. Also cannot find a backtrace for the same reason now.
From the reproducing early today I remember that restarting Emacs doesn't help - or is at least random, since now a newly started Emacs doesn't exhibit this problem. It feels like the problem is in some cache and the cache appears to be persistent (cross-Emacs), if TRAMP uses something like that at all.
The files I have noticed this on have names like "/ssh:USER@HOST#22222:/...", i.e. using SSH backend and a non-standard port, if that matters.
Paul