Oh okay please let me know if you have an update, because I think your situation is very similar to mine. My mvno is 'US mobile'. Thank you for your time. I'll try to contact to Tmo support if they can unlock my sim.
I was able to get unlocked today.
I got a reply from @tmobilehelp on Twitter saying "we escalated the request to the unlock team. They received an error during unlock. You have to talk to the manufacturer". Of course OnePlus says TMobile will have to unlock the phone (but OnePlus is supposed to reach out tomorrow, the guy said he would check into other methods for me to bypass TMobile. Not expecting anything)
Then I asked her if she could try to submit it again because I was on the network now and wasn't on the network the first time I asked (I have an active account but my sim wasn't in the 9 Pro).
She asked me for a full receipt instead of my edited receipt so she could submit it again showing proof I own the phone fully.
While I was getting that documentation ready, she flagged me down in a Twitter DM to say:
"Upon further investigation into your IMEI, we were able to approve your unlock request. You are now able to unlock via the in-phone unlock button!"
She said "your phone comes back as 'purchased directly from TMobile', which is a different process. I did not realize this at first but now your request is done and will unlock"
You may get lucky finding her on Twitter again and going for the same story. She knows how to get it handled. Her name is Yecenia. When she's active on Twitter, you will see her replying to people complaining @TmobileHelp with "^YeceniaFlores" as the signature (or similar). Maybe ask for her to help if you get another agent. Explain that you own the phone and want to unlock it for development, offer to show proof of purchase and see what happens