Form 8621 error

PS has never been great with international returns and even Lacerte/PTO have issues with F.8621.  Intuit just doesn't pay a lot of attention to these.

Sounds more like a weakness of PS than a bug but I could be wrong (since I don't use PS).  Unfortunately, no one here will be able to help you since we're all just peers of yours and don't have any sway with Intuit. 

If you believe it's a bug, you will need to contact Support directly.  Otherwise, there are two things you can do:

1) Register this as a product idea and let others vote on getting Intuit to implement it in the distant future at https://proconnect.intuit.com/community/proseries-tax-idea-exchange/idb-p/603; or

2) Write your wish on a piece of paper, fold it neatly into an airplane, light it on fire, and toss it out the window (just not toward your neighbor's home).

History has proven that either option would be just as ineffective.  You have an equal chance.  The choice is yours.

Good luck - I mean it from the bottom of my heart!