Recently opened a couple maps created in earlier snapshots for 1.16 and, while updating them by running around the terrain they cover, watched my changes render and the whole area get slashed with odd, short east/west lines of pixels, some seemingly random types, mostly gray. The east-west nature of these short lines, apparently casting shadows, will rotate with the map when placed in a rotated item frame.
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Thank you for your report!
We're actually already tracking this issue in MC-179858, so I resolved and linked this ticket as a duplicate.
However, that ticket has already been resolved as Fixed. The fix will arrive in the next version or is already included in the latest development version of the game.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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Thanks for the quick response, I'd searched for "maps" and only saw reports of white outlines around maps in item frames. Glad to hear this will be fixed soon.