I had something similar happen pre-pandemic for a reservation either in China or Singapore ( I can't remember which exactly). The agent who handled it also said it was correct on the back end. They were standard 35K certs from my AX. Agent was incredibly helpful. It took a long time on the phone, maybe an hour, possibly 2. There was a 35K point difference at the time. IIRC, nobody could sort out the IT issues (this was 2019 or before) and he ended up gifting me 35K in points, then doing the booking on points and then reissuing my certs to me as new certs, which meant they had a brand new expiration date and 12 months to use them. It was lovely to have extension of time and the same expiration on all of them. He asked my permission before he did all that, but it worked for me.

I was very glad to have a confirmed reservation when I landed in Asia and not have one more thing to worry about. The extra 12 months on the certs did not hurt either. YMMV.

Mostly posting to let you know you are not alone and that the problem appears to be several years old. My advice is to keep escalating it with customer service until you get someone with authority to fix it or give you a work around. And be patient with them. It is apparently a bug in the system, and we all know how Marriott's IT is, e.g., not great.