It took me ages to find someone else with this bug so I was beginning to think that I was the only person experiencing it. Unfortunately the bug still isn't fixed in 20w18a, so hopefully they have the bug fixed by the time the full release comes out.
I believe the bug only happens when you try and use a map on a chunk that was loaded up on a previous version of the game. Whenever I try and use a map on my house I get the bug (because the chunk my house is in was loaded up in 1.15.2), but when I tried going to a completely new chunk that I've never been to before (so it was loaded up in 20w18a), the map works perfectly.
yeah, you really shouldn't convert your worlds to snapshots. they can easily corrupt or get messed up. if you wanna test out new features, make a new world. if you really wanna use new features in a current world then wait for a full release of the update.