I found this bug with maps in the snapshot that would show random markings that weren't in the world, and it's been around since 20w20a. I've checked in the other dimensions and it only seems to not happen in the nether, it happens in the end too.
[media]
MC-184697
Map glitch
Resolved
Duplicate
0
charli m
(Unassigned)
Unconfirmed
(Unassigned)
20w21a
Retrieved
Duplicate of MC-179858, please read the moderator note on that report.