Hmm, it could be a bug but it should be possible to enter the doors. Just don't dash toward the door otherwise you will just change the mode.
Viewing post in Mirror Rogue jam comments
Ah indeed it was my fault, I didn’t noticed that only one of the door opened and I must have tried the wrong one, did a short run and managed to get 18 kills before dying, the game sure isn’t easy especially if you waste a bit of time and the enemies start accumulating!
I’ll need to try again later on to see if I can manage to go further