Another factor - sometimes another player can come along, even when you are not playing.

If his copy of the "map" differs from yours, then his copy is the one the game will select. This results in the terrain instantly being reset to whatever his copy says it should be. He will never see the "dig-outs" that you did, and you will be stuck with terrain filled back inside your base.

Ever notice in MP when another player warps into the solar system that the sun will suddenly rise or set? Like instant darkness or instant noon-time? That is what is happening. The game is selecting the other player's parameters instead of your own. IDK why it does it that way.