Did you try hdmi_drive:0=2 if you are using the 1st hdmi port in config.txt? I have not seen anyone reporting this. It may not be getting the right EDID info from your monitor.
Did you install the Pi OS patched mesa?
https://forum.manjaro.org/t/switch-install-from-rpi4-to-rpi5/150632/39
I would stay on the unstable branch at least for a while to get the latest fixes as they do them.