The "complete darkness" rule for spawning glow squids seem to have been unintentionally removed when the spawning rules for glow squids were changed in snapshot 21w40a.
How to reproduce:
Create a creative default world in one of the affected versions.
Locate an aquifer below height 30 using spectator mode.
Fully light the aquifer with lanterns (or other waterloggable light sources), and make sure that none of the water blocks are completely dark).
Move away and wait for glow squids to spawn in the area.
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Another thing is that glow squids aren't restricted to spawning in water source blocks. They can spawn in any water block as long as it occupies a full block. That's likely why they spawn very commonly in vertical waterfalls.
Note, this was changed in 22w07a, despite this not being considered a bug – glow squids now only spawn in light level 0. See also MC-238977.
The spawning rules for glow squids and axolotls was changed during the snapshot phase, so this might be intentional. However the fact that they are able to spawn in any water, including bodies of water that are tiny (see e.g. MC-242482) is a legitimate issue in my eyes.