Buried Treasure Map Data Wiped

While making a wall of maps zoomed out by one, I returned from filling two maps to find that a Buried Treasure Map that had been fully filled became blank. It was on the wall at the time. The blank version also did not have the usual lines on a Buried Treasure Map to denote land and sea.

Duplicates that I had of the Buried Treasure Map were unaffected, and no other map on the wall became blank.