light level raised by redstone lamps was not properly resetting when lamps were switched off.
In both instances where I reproduced the bug I was in the nether within range of a monster spawner. (although this may have just been coincidence as the light level bug seemed to be focused only on certain blocks.)
the light level can be reset to 0 by re-logging the world. but returns when the lamps are toggled
included are screenshots of the issue [21.41.21], the (correct) light level after re-logging [21.42.32], and while redstone lamps are active [21.42.37].
Also inclueded is a wider view of the effect [21.41.49]
I tryed 3 more times to reproduce the issue in differn't locations/ worlds. one was sucessful (screenshot 21.56.40).
World seed was -216290396724398692
Co-ordinates are in the screenshots.
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Invalidating as you're using a modded client.
If this issue also exists on an unmodded client, please create a new report.