To explain the bug, I have to explain the situation a bit. Basically, I have this old world that I've been working on for many years. In the beginning, the world was a superflat, I eventually changed it to a customized world generation and when that got removed in 18w06a (1.13), I changed it to the default world generation, than to amplified and then back again to default.

Now for the bug, there are some gray lines that appear on some of the old maps I have in the world. The bug seems to affect only the older parts of my world, the ones that used to be in superflat. The grey lines only appeared after taking the map in my hand (when it updated). The size of the map does not change the problem, there are still grey lines on bigger maps. I've only noticed this bug happening in 1.16.1. The world has seen a few of the Nether update snapshots if this can help.

See pictures for more information.