OK, I can confirm that I am only able to reproduce this during a thunderstorm. Not during regular rain. Come to think of it, I remember the clouds also indeed being dark like during thunder, so yeah, definitely thunderstorms. Is this intended or...? 😉
Oh, maybe I have missed something if this doesn't occur when others try it. I don't know if it was a thunderstorm, but for the (very) short time I was outside of the savannah I didn't notice any thunder. I should have checked how the weather was outside the savannah for longer then a few seconds... oof. Just when I thought of doing that shortly after noticing the problem, I went to bed... so that made any rain go away. ¯_(ツ)_/¯
Ya know what, I'll try to reproduce it in a test world; maybe at the same location in a copy of the world, too, to check if it might have been the shadow provided by the surroundings--despite earlier being convinced that it couldn't. 😃
But why is it intended to have the light level be low if there is thunder nor storm in the savanna? At this point I'm only just curious to know how the devs were thinking here. 😉