Upgrading all gnome-shell packages to 43.3-1 and rebooting seems to have solved the issue, no more focus stealing occurs anymore.
As it seems, the mutter packages on my machine had been upgraded during my after-login-upgrade routine, and I had not logged out and in again yet. I guess that would have helped already. Probably now-irrelevant additional information: the focus-stealing applications were all installed from distribution packages; no snap/flatpak/etc involved. Thank you – again!
