@DaMaloma: That's it! When I place a water source on one of the lower layers, it is replaced by flowing water. Guess my issue is just a symptom of the issue you linked, then.
The question is, why doesn't this happen in the above-ground pool?
Oh, seems I was half-wrong: The issue only appears when I dig a hole and fill it with water, not when I build up a pool above ground like you did.
[media]PS: How do I attach a picture to a comment like you did?
You're right, that's it. I didn't even consider that filling upwards in layers and filling upwards in colums could have different results.
Thank you very much, it's nice to finally have an explanation. 🙂
Hope this isn't too hard to fix.