Because I didn’t manage to fix this important issue, even with the help of the Manjaro community, I decided to switch to another Linux distribution that is more stable. The caveat is that stable distros often have old software. But with Flatpak and Snap this problem (hopefully) can be circumvented.
PS: I took this decision before the most current stable update came available ([Stable Update] 2021-12-10 - Kernels, Systemd, LibreOffice, Plasma, Browser, Thunderbird - #2 by philm), so I do not know if this could fix this issue.