It probably depends on where /e/ acquired the phone from. Europe firmware? N. America firmware? Global? You could ask /e/ Support about that, but if the IMEI is approved by the carrier, you should be good to go: https://en.wikipedia.org/wiki/International_Mobile_Equipment_Identity#Structure_of_the_IMEI_and_IMEISV_(IMEI_software_version)
I’m not sure why Mint Mobile would disallow it if T-mobile does allow it. Maybe Mint is afraid their coverage for your address isn’t as strong as T-mobile’s…I’m just guessing, though.
Edit: Good news is on the horizon: