I just tried and the behaviour is the same.
I have completed the reassignment for my org and basically this is my observation..
Scenario is to assign data from userA to userB.
After pressing reassign,
1. if userA has data to reassign:
a. there will be no dialog pop up
b. (via AdvancedFind) we know data are being reassigned
c. for each completion of 1 entity reassignment, the user form will pop up "An error has occurred." (see 2nd screenshot above)
d. after all data has been reassigned, there may or may not be another pop up "Reassigning Records..." (see 1st screenshot above).
2. if userA has no data to reassign
a. the pop up "Reassigning Records..." will appear immediately
b. this pop up will stay on browser indefinitely