I cannot for the life of me get the 32nd box to appear. I'm having this issue on both Sword and Shield, and across a couple of different profiles on the same switch. I've tried placing a pokemon in every box and coming out of, and back in to the boxes. Nothing seems to work.
The only things I haven't tried, is completely filling every box... or, releasing every single pokemon, and then going through the process of filling the boxes again...
Is this an issue that pkhex can cause? Its possible that I have in the past, put pokemon in to box 32 using pkhex before it was visible, but currently it is empty (in pkhex).
Edit - I've already beaten the story and caught Eternatus.