SUMMARY

When battery reaches the "critical" level, the screen brightness is automatically adjusted (in the default configuration). If the brightness was already low, this can cause the brightness to be raised, which is exactly the opposite of what you'd want when battery is low.

This was apparently already fixed in the 4.x days (#198463) so it's a regression in Plasma 5.

STEPS TO REPRODUCE

On a laptop:

1. Use default settings
2. Set your brightness to the lowest level, say 5%
3. Wait for your battery to drain to the "critical" level (10%)

OBSERVED RESULT

The screen brightness is raised automatically to 25% (the default setting for "critical" battery).

EXPECTED RESULT

Brightness is never increased. It could be reduced by a factor instead (so 20% would become maybe 10%), because luminosity changes are a very good indicator that power is low.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kde Neon
(available in About System)
KDE Plasma Version: 5.21.5
KDE Frameworks Version: 5.82.0
Qt Version: 5.15.2