Upon zooming out a map, it registers weirdly and leaves lightly-coloured stripes throughout the map (land-based areas).
However a square-shape area of the map is displayed and it moves whenever the player moves, and any stripes that weren't there before appears again.
Edit:
This only happens when the render distance is set to 4 chunks.
My friend and I bought an ocean explorer map and when we reached the location, the ocean monument wasn't there. We then bought another map from another villager in another village, and there was the same issue. We don't have similar issues with the mansions.