This is a difficult problem to solve but does occur (rarely)

The issue in the *A system if the operating carrier has to "pay" UA to post the miles in MP. This means the operating carrier needs to notify UA and accept the "charge". Once the operating carrier has posted the miles (perhaps due to an error by their agent) to their program, the operating carrier is very reluctant to undo the process and credit to UA MP. It is not UA refusing to credit, it is the operating carrier refusing and not authorizing UA.

This can sometimes be fixed but it takes persistence.

You need to work with the operating carrier and get them to void the credit to their program. Point out you have no membership in their program and it got posted to the wrong account.
Once the credit has been voided, you go to UA and ask for missing mileage credit.

There have been recent issue with AC when you try to get seat assignment, the crediting carrier has been changed to AC
United flight incorrectly credited to partner airline (AC)
Perhaps something happened in get TG seat assignments???