Ethan,
Please try this build:
https://www.camerabits.com/download/PhotoMechanicPlusR6140_5a7e8298.dmg
Note: it's a work in progress (ignore all UI having to do with external proxies) but it should otherwise solve your issue.
-Kirk
Hi Kirk,
The dialogue still appears, quickly flashing on and off the screen, and it takes a bit longer to add photos to the collection than it does to remove them, but the issue is fixed.
thanks,
ethan