The end is extremely dimly lit while the ender dragon fog is present and it is raining or thundering in the overworld.
See attached video for an example (public viewers will need to use the logged-out view due to META-212).
If the recommendations I made on MC-303836are implemented correctly, this issue will be resolved.
Update for 25w45a: It seems that the presence of the effect is now determined when the end fog is first applied, instead of continuously.
Steps to reproduce in 25w46a
Create a new creative world
/weather rain/execute in the_end run teleport @s 0 72 0
✅ Everything looks fineRelog
❌ The rain overlay now shows
This is what it looks like with night vision.