This bug was appearing earlier on in the 1.16 snapshots, before the pre-release versions (around 20w21a I believe), and seems to still occur in version 1.16.1. I took a map off of an item frame to update it, and this caused the map to become corrupted-looking (as pictured below). In 1.16.1, this seems to happen with maps created before I updated to the snapshots, and also happens with newly crafted maps in 1.16.1. Strangely enough, it sometimes effects only small portions of certain maps, but always happens when maps are updated.