For some, the biggest advantage of OnePlus phones isn't the price or the software, it's the fact that they're one of the last manufacturers offering easy bootloader unlocks, which makes rooting and ROMing the company's phones pretty easy. Unfortunately, the unlock process that's been put in place for the T-Mobile version of the OnePlus 8 5G doesn't seem to be working for many device owners.
Reports aren't especially widespread (odds are most folks trying to unlock their bootloader didn't buy the T-Mobile carrier-branded versions), but they are consistent. Customers that secure an unlock code from T-Mobile are met with an invalid IMEI error when entering their information into the company's unlock token form to continue the bootloader unlock process.
This isn't the first time OnePlus device owners have run into an issue like this with the carrier-branded versions of the company's phones, either. The 7T Pro Mclaren on T-Mobile ran into similar issues at first, though they ultimately resolved within several weeks for most. We don't know what the cause of the problem for the T-Mobile OnePlus 8 5G might be — for all we know, this could be an issue on T-Mobile's end — but it could be a similar problem that we'll see resolve in the coming weeks.
We reached out to OnePlus for more details or an explanation of the issue, but no response from the company was forthcoming at the time of writing. In the meantime, those picking up a OnePlus 8 intending to immediately unlock the bootloader may want to avoid buying the T-Mobile version of the phone. (So far as we can tell, the Verizon version isn't bootloader unlockable at all, so you might just want to steer clear of all the carrier versions of the OnePlus 8 if you hope to root or ROM it.)