Why is unlocked iPhone on T-mobile network displaying AT&T sometimes?

I have an AT&T iPhone that was unlocked by AT&T for use with T-Mobile. Usually, in the top right of the screen, "T-Mobile" is displayed next to the signal bars. Sometimes, however, it switches to "AT&T."
Why does it do that? Is that OK?

Tmobile uses ("rents" lmao) approxiamtely 63% of ATT towers in the USA…

you are obviously connected to an ATT tower on tmobile signal… Plus your APN settings may still contain ATT data… This is why your alpha tag is displaying ATT and not Tmobile sometimes…

its normal… Its not gonna kill the phone or you or rip a hole in the space-time continuum…

The reason why it displays an ATT is because TMobile shares a tower with ATT. So no need to worry about it.