Starting February, the following (new kind) of error message has shown every time when I applied SAUA on United.com with a BR-issued B class transpacific ticket:

Your paid booking class is not eligible for upgrades.

Once I removed the ticket number and reapplied, this is the error message:

The traveler has the same name as an existing traveler on this flight.

Had to do 24-hour cancellation a few times due to above error messages. Or just paid more to be on Premium Economy.

The good thing I found out this month is that once the ticket is cancelled, the miles used for SAUA are returned from BR to UA almost immediately. It was not the case before this year.