Isolated water block floating in mid-air

The bug

World generation creates an isolated water block that floats in mid-air. From the height it looks like it's not a full source block, but when updating the area (e.g. by placing a torch), it spreads water from it.

It seems to be a permanent problem since I already had it when I generated a new world with that seed in previous pre-releases (and when converting a 1.12.2 world without that chunk with the same seed).

How to reproduce

Seed: -4270258239336733475
Coordinates: /execute in overworld run tp @s -188.92 64.00 -387.83 33.98 47.65