I think I have found the real culprit.
For reasons I do not know (they did not appear in the list of orphaned packages, and when I removed them, no other package was removed as a dependency…), I had form some AUR these packages:
[2024-05-28T19:35:26+0200] [ALPM] removed qgnomeplatform-qt6 (0.9.1-8)
[2024-05-28T19:35:26+0200] [ALPM] removed adwaita-color-schemes (0.9.1-8)
[2024-05-28T19:35:26+0200] [ALPM] removed adwaita-qt6 (1.4.2-1)
[2024-05-28T19:35:26+0200] [ALPM] transaction completed
…and now the apps are not so nice-looking, but they work. So I think @pux was right.
Thanks again, I am learning a lot from this thread.