Maps show stone-like pattern despite it not being there

For anyone who opened their world in 20w17a and is still experiencing this issue in versions after 20w17a:
Try optimizing the world:
Important: Depending on the world size this may take a while.

• Client:
  1. Select the world on the world selection screen
  2. Press the "Edit" button
  3. Select "Optimize World"
  4. Check "Erase cached data"
  5. Press "Create backup and load"
  6. Revisit the area located on the map to update it
• Server:
  1. Create a backup by copying the world folder
  2. Start with --forceUpgrade --eraseCache

If you are still experiencing this issue after trying the steps above, please comment and attach the world (in the version before you opened it in the latest version of the game) as zip file.