The water flowing downwards on the right of the screenshot attached was likely because I snagged water out of the 'lake' with a bucket. When it didn't replenish like it should, I went underwater to investigate and discovered this bug.

I have seen several other lakes in sand mesas and not seen a bug like this.

EDIT: I take that back, there's a long river that runs through the biome and it is the same way.

I did plenty of searching to see if there was a similar bug submitted for this and didn't find anything that looked related.

EDIT2:

Coordinates:
X: -692
Y: 63
Z: -257

Seed: 279699790638812385

I checked the remaining water around in the Mesa and all were similar.