For me, mesa 21.1.4 finally fixes the problem! For everybody who is interested in testing it:
sudo pacman -Syusudo pacman -S yayyay -S downgradesudo downgrade --ala-only mesa- Select option
102formesa 21.1.4 - Reboot to apply changes.
Find the changelog including “our” bug here: Mesa 21.1.4 Release Notes / 2021-06-30 — The Mesa 3D Graphics Library latest documentation
Thanks to the mesa, firmware, linux kernel, and manjaro developers! Maybe we can support them by donating a coffee?