I recently bought some ocean explorer maps from a cartographer villager in 20w20b, but the maps which were given to me acted like normal blank maps, even though they were still named and textured as ocean explorer maps.

The arrow that shows the player's location was invisible on the map, and so was the monument icon. I was able to zoom out one of the glitched ocean explorer maps (crafted with 8 paper), which should have been impossible. 

About a month ago, I purchased a woodlands explorer map from the same villager, and everything was normal. However, when I buy a woodlands map from him now, I experience the same bug as I do with ocean explorer maps.

The problem persists even when I upgrade my world to Pre-2. 

I am aware of a similar bug which occurred around 1.14, but my world is relatively recent (created after 1.15)