Very easy to reproduce. Leaving any map (even just a TP back to drop off items when your inventory is full) then return. You will have lost most of the map layout that you already uncovered.
It also re-spawns all enemies as well, which is a pain but not as annoying as losing map knowledge.
This is the ONE thing that really annoys me. I’m hoping it’s a bug, but feel that it’s not, and is meant to be like that. It thoroughly kills any idea of exploration like all the little hidden areas in the game, it is also extremely frustrating when an area has to be revisited for another quest.