Thanks for picking this up. I bought the Japanese raw tankos and the French translated manga (it's called Dédale in the DokiDoki edition) thinking I might scan it in and translate it myself this winter but you've saved me the effort.
Maybe the monster only shows up once they get to a new area, kinda like how in actually games the old locations would be removed once you went through them. So my guess is that they don't need to worry. (Page 20)