Does the map look like something you cleared in a previous game, or is it just a chunk of the shroud cleared where you have never been?
its my first game and profile so no previous exploration.
I'll attach a screenshot when I'm at my private PC again.
I think it's around the graveyard, you suddenly unlock a large portion of the map to the north (till the teleporter there beyond the Arhu Abomination Cave).
Pretty sure it's been that way since beta, it's because of the extreme height difference I think (you can see all the way there, so it displays all on the map there).
Just my theory...
if this happened at the graveyeard it would've been at some point in this map taht is close to the western gate, as I just walked into the first Zombie group and then decided to do more town-exploration.
another area is visited right before that happened was the one to the east (with the vast toxic area) I set that area on fire, went to the portal on the cliff above the starting beach and returned.