New one popped up for me today. Business Class award booking, all United metal. Mix of domestic and Intl flights.
When selecting seats during the check out process, I got the below screen when attempting to select Polaris Seat. Domestic flights prior to this one - seat selection worked fine.

I clicked the "Continue to upgrades" button just to see what would happen, but the box just disappeared. Trying again, and same pop up box.

I completed the purchase, then went back to manage the reservation. Was able to select my seat with no issue.