Form rules changing user to the same user

Using a form rule that changes a user field causes form lock up when the field is changed to the original value.

I have a field where I am setting a manger for a project based on categories - so manager A manages projects in categories A1,A2,A3 - manager B in B1,B2 B3 etc.

If i have a form rule saying 

On category update - change Manager (type=User) to Manager for new Category

it works perfectly well if I always change from A1 to B2, B2 to C3, C3 to A1 etc
if I change from A1 to A2 that won't change the manager (but all rules appear to be processed okay) but subsequent to this no manager changes will happen.

So if I change from A1 to A2 to B3 the manger stays at Manager A

I'm trying to identify whether this is a bug in Quickbase or my logic is incorrect.

If I create rule logic that does:

manager change needed - change manager to <blank>
manager change needed - set new manager 

It will work perfectly okay with the A1 to A2 to B3 scenario.