While testing a new mapper, I discovered distinct banding artifacts in some underwater terrain.

The confirmed bands run in an east-west direction and are always at the same y coordinates modulo 4. This causes them to run in straight lines across the map. In bulk, the bands show up as clear straight lines running across the map if the map is shaded to highlight differences in underwater terrain elevation. Up close, the bands can be seen as distinctive flat underwater cliffs. These bands are most obvious underwater near deep ocean islands.

North-south bands are strongly suspected to occur with similar behaviour. Consider these to exist as well.

The screenshots show the issue. The terrain is shaded so that terrain is darkened as if lit from the north. It is implemented by reducing the brightness of a pixel by about 5% for every block the north block is higher than the south block.

Map seed is 13.

Screenshots:
(1) Mapper output showing the terrain banding. Top left has coordinates -1024, -1024 and the centre is at the origin.
(2) Closeup of underwater terrain at (174, -764).
(3) Closeup of underwater terrain at (238, -748).

This banding has also been confirmed in 1.12.2 and in 1.8.9.