One of the recent updates seems to have reintroduced the issue where enlarging the size of an explored map (zooming out) renders the new larger map blank. Previously documented in 16948 and 25099 (and at one point declared to work as intended, but then fixed to work as expected).
I haven't checked all the permutations, but I was bitten on a Locator Map 2 Level 2/4, enlarging it to Level 3/4 via an Anvil. I had not too long ago enlarged this same map from Level 1/4 to 2/4, so it's probably 1.6, 1.6.1, or 1.6.2 that reintroduced the issue.
It is perplexing how the devs think that deleting traveler data from a map when expanding (zooming it out) IS perceived as intended behavior. It can't be THAT hard to expand a map and have it keep any previously traveled data. When I started playing and would only have enough paper to expand an initial map to level 2, this would really irritate me when I finally got around to expanding them to level 4. This would a great thing to revisit.