Oh, as an addition to this that I just spotted, if I try to move Spencer Prior with me, it's offering him the first team assistant manager role, rather than the U18 one that he is currently employed in.
I'm experiencing the same issue while trying to move from one club to another. As best as I can tell, it is because some of my existing staff are classified as "Unknown" (see screenshot) rather than grouped into the area they're working in. If I try to add Tommie Hoban into moving with me at all, the confirm button is immediately greyed out. There is space for him in the existing U18 set up at Bristol, and it makes no difference if I choose to remove any of the existing staff.
I've created a save file at this moment if that would be helpful