Not sure if this should be a new report or included as part of MC-219511, since the initial steps were the same.

I had a large map of the area around the centre of the world displayed on item frames. I went to the Nether and came back, and the central map (the one containing the 0,0 coordinates) failed to load, and simply displayed a blank scaled-up item frame texture.

When I used Creative pick block to try and get a copy of the map, it loaded the image shown in the screenshot. It retained the banner markers that had been previously added, and the shape seems to be roughly based on the terrain (given the blank spots where the ocean is). I suspect it's somehow trying to read the locations of blocks starting from a much lower altitude than normal.

Any new maps that I created subsequently (of this central region or any other location) also displayed similar strange grey textures. Reloading the world didn't help.