My understanding was that the corflute bottom board should be in the top position initially, then moved to the bottom position once my nuc seemed settled in. Now some of the bees seem to be using this as a backdoor. Is this a problem?
It could be as the bees may be trapped under the gauze and will die there, are you able to seal up the slot to prevent bees from entering with some tape?
I think a better use of the corflute is to leave it in the upper position unless you are trying to trap pests like wax moths, varroa or SHB. In that case, coat it with vaseline or cooking oil spray and put it in the lower slot.
If you have bees using it as an entrance, you probably have a distorted screen in your SBB:
That gap is important for some of the guard bees to go down and get rid of or ‘herd’ the beetles and mites that were pushed out of the combs. I use a small piece of board to block off the entrance if I see bees are getting stuck on the beetle trap. Stewart did a video of a beetle trap using a piece of table cloth with a fibrous backing that the beetles and mites get stuck in. This is attached to the board by means of a double sided sticky tape. But either way, I believe that gap is necessary, but block it off if you see the guard bees getting stuck themselves.