Fish spawn in absurd amounts in underground waterfalls

Summary

I placed a water source over a big drop and got 3 stacks of fish (cod & salmon) in about 5 minutes. Fish rain from waterfalls. If it's relevant, this was under an ocean.

To reproduce

1. Generate a world with default settings and the seed 3220264758811034948 in 21w37a.
2. Go to -20 0 50 and place a water source block at the roof of the cave.
3. Wait at the bottom of the waterfall.

Screenshots