Steps to reproduce

1. Build a platform out of solid blocks with glowstone every 5 blocks in an alternating pattern, as shown here:
2. Set time to midnight.
3. Set weather to clear and wait a while.
4. Set weather to thunder and wait a while.

Expected result

Drowned do not spawn in the water because the glowstone lights all of the water to light levels from 9 to 13, and thunderstorms should affect only sky light levels, not block light.

Actual result

Drowned spawn on all of the solid blocks (though not on the glowstone itself) during thunderstorms.

Note

More general testing described in this comment.

Original summary
Drowneds Spawning In 1-Block Deep Waters With Light Level 8+
Original description
Recently, drowneds have begun spawning in a large pool of water that should be drowned-proof and has been working since 1.16.40. The waters are 1 block deep and I've arranged the glowstone on the floor of it using a certain method to keep the light level of the waters to be at least level 8 at all times so drowneds shouldn't be able to spawn. The glowstone in the 1 block deep waters have been arranged to keep the light levels in the waters to be at least level 8+ at all times. This does work because during the day and night, drowneds don't spawn there. In fact, I've only seen them spawn when it thunders. So I did some research. Thundering is said to lower the sky light dramatically, allowing mobs to spawn in unlit areas during the day. The problem is, the internal light levels of the waters are already enough to stop drowned spawns (since they only spawn in levels 7 and lower). The decrease of sky light from thundering shouldn't be able to affect the internal light level of the 1 block deep waters from the glowstone as I've already said, the light from the glowstone arranged in this pattern already provide enough light to the waters to prevent drowneds from spawning during the day, night, and rain. I'm assuming it definitely has something to do with thundering and the effect sky light has on mob spawning.