The bug
In the pictures below, you can see that the water is not still, although it should be. It looks pretty weird and doesn't seem to happen at the edge of oceans.
How to reproduce
World type: Single Biome; Lush Caves
Seed: -6478590947986357930
Coordinates: /execute in minecraft:overworld run tp @s -83.99 49.00 403.13 -395.40 13.50
I think this is caused by MC-213789.