Still makes no sense, but they intentionally deemed this taxable despite clearly recognizing the underlying contract as a Roth. I cannot locate any such IRS guidance, and there is nothing to this point in the 1099R Inst. They could be right, and may have received a specific document from the IRA on this exact situation, or others like it given their massive annuity business.
If so, your efforts to file an 8606 or 4852 may not be accepted by the IRS. The 4852 would probably be the way to go as the Q code would eliminate any taxes or 8606. There might be more to the story if this is actually an out of contract payment, and if so perhaps they should have issued a 1099 MISC instead of an R. Any action you take becomes a crap shoot since there are so many variables involved and the TIAA explanation is not really complete.