I got a mine floor that had no exit. I blew up every single rock, plant, everything, and nothing happened. I saw one plant that looked very out of place but couldn't do anything with it. See attached iamge
I've had that happen too. It seems to be that in some infested floors the stairs that will appear after you kill the last monster appear at the ladder from the previous level... and sometimes it appears where you kill that last monster. If it wants to populate the stairs where you killed the last monster, but the terrain at that location is not able to have stairs populated there, it just fails to populate anywhere. I've encountered this a few times. I typically just leave the mines and re-enter as as a work around since a new mine is generated every time you enter.