Map reiniciates in caves and town portals.
Sometimes, when I go out from a cave, and I come back, all what I have discovered, is no longer discovered, and I begin as I had not discovered anything. This happens in caves, or if I use a town portal (not the “z” key, I mean the circle on the ground).
Pretty sure that’s not a bug. Like most games of this type (multiplayer games with random maps), the world is not intended to be persistent. Although frankly I too would prefer it if it were, particular for a solo player.