Unlocked, but not??

I finally paid off my iPhone 12, and it’s unlocked, as far as T-Mobile can tell, but I can’t switch to a different carrier, because it’s still showing Carrier Locked on new carrier’s end AND on my end- in my settings. T-Mobile reps only can see that the phone and device/ sim is fully unlocked. When I log into my account and check the “device lock status” it says Unknown Status and in small print underneath device not recognized. T-Mobile rep and her supervisor tried to tell me that I have to take it to Apple for unlocking, luckily a Spectrum rep was on the phone with me and reminded the T-Mobile rep of the proper procedures/ next steps. So, now I’m awaiting a response from the tech dept or something where she had to submit a ticket to request a manual unlock. The whole thing seems sketchy. 
I bought the phone through T-Mobile (in the US), have only used this phone the entire 30 months, and have remained in the country. What could be the problem??