Thank you for your responses.
The issue was just a manual W2 entry error on my part, unrelated to the subject line above. I selected AA (uncollected social security or RRTA tax) instead of A (Roth contribution) and this moved the dollar amount in this box to taxes still owed. The number was within a few hundred dollars of my social security + medicare withholdings, so I assumed that the employer portion of these taxes might be the issue. This was a red herring.
