Ever since 20w17a, creating a map in older worlds results in scattered lines over the top showing the layers underground.
What I expected to happen was...:
prior to 20w17a, the maps in a pre-existing world worked perfectly fine
What actually happened was...:
when 20w17a came out I created a map and it gave me the glitchy map texture in the attached image below.
Steps to Reproduce:
1. go into a pre-existing from 1.15 world using a current snapshot
2. create a map
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migrated
Of course, worlds that were opened in 20w17a are 'infected' with that bug. That's the reason why you shouldn't ever play a snapshot and join your normal singleplayer world. Always create test worlds.
This is going to be annoying for my level 4 map, maybe it won't affect it if I kept it in an item frame
For worlds created before 20w17a and are opened in 20w17a, the heightmap can be fixed by selecting the affected world, clicking 'Edit', then clicking 'Optimize World'. Ensure that the 'Erase Cached Data' box is ticked, then run the optimiziation (I'd recommend clicking "backup and load" over "I know what I'm doing", but that's up to you). In my experience, this solved the issue on multiple affected worlds.
violine1101
See MC-179858 for instructions on how to repair your world.