Plasma 6.2 introduced a lot of new code to handle different brightness levels on different screens, and unfortunately one result of that is behaviour changes and some edge cases that get triggered occasionally and unstable behaviours.
There are a few tickets already open that track brightness regressions:
- 482713 – DDC-based Screen brightness control randomly unavailable in some sessions
- 484663 – Switching from low battery mode to AC power should restore previous screen brightness
- 476550 – After DDC-capable monitors become dimmed, moving the mouse raises them all to a single brightness value, not the brightness they had before being dimmed
- 482278 – Screen remains dimmed after unlocking
(These are just the confirmed issues - there are a few more that are in the “reported” status, which means a developer had yet to look at them).
I suggest tracking these - they should be resolved in the 6.2 bug fix releases as one of the developers that works on these - @jpetso - is pretty active.