In my opinion, this is a known issue at this point. I experienced this issue twice and seen reports about zero luggage allowance on United award ticket on TK metal.
1-) Couple years ago I booked Business class ticket using United miles, while checking in for Return AYT>IST>IAD they said luggage allowance shows zero. Supervisor got involved and the issue was fixed quickly.
2-)In June I had another United award ticket for Economy and again during check in, their system showed zero luggage allowance. This time it took even longer to solve the problem and check 2 bags. What is interesting is, if you call M&S they are able to see and verify luggage allowance (they said its total 70KG, I'm TK Elite) but ground staff cannot verify this on their system.
I could imagine this could be quite frustrating.