Some maps that are displayed on item frames get empty.
Bellow there are 2 screenshots with the maps on the snapshot 20w20b,and the empty maps on the pre-relase.
Optimizing the world doesn't fix the problem (I had a rendering issue with maps in the previous snapshots but optimizing the world fixed it).
Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-185417.
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