I'm not sure which of these events may have caused this, but the map reveal progress was reset on my particular PS4 game.
Normally I host the game from my PS4 and play with one other person.
Sunday night, I downloaded my save on my friend's PS4 and we played local co-op.
Monday night, we both installed the latest Don't Starve Together update, I downloaded my save from PSN and hosted the game as normal.
When we were in-game, the map progress for both of us was reset.