How do you freeze them? Do you take off the peel and chop them? Otherwise I don't know, never had problems with that.. Hope you'll find a way! :)
i assume you peel them first? (no need to chop them i never do that)
Anyway maybe you have that kind of freezer (energy saving or whatever) that doesn't notice the temperature change that the bananas bring causing it step the cold up a notch.. i don't know if that's actually a thing but i can remember i suspected the newer freezer that we have inside the house here of doing that, cause thing wouldn't freeze or something, but my bananas are always in the freezer in the garage.
sometimes when i fill my freezer up too much my bananas don't freeze fully, they remain a little soft. I think its when there is too many things in the freezer, the air can't get to everything evenly? Im not sure why it happens but when my freezer is less full the bananas freeze fine :) maybe just try not putting so many bananas in the freezer at one time.