It's really difficult to work like that.
At the moment if we have a multibody, with many bodies, and we need to realize a new body Fusion choose to set CUT to all bodies. No matter what you want to do (join or cut only a body).

This leads inevitably to freeze Fusion with no really a reason...
then choose if cut or join and what bodies (and this often is not simply just with names..) are involved in the operation.
Solution:
Just choose Join/Cut
THEN
choose what bodies are involved (especially for the cut operation).
Please!
Thanks!