Using ProtonVPN with T-Mobile App on iPhone

I switched to T-Mobile yesterday. All my apps work fine on the T-Mobile network with ProtonVPN except their own T-Mobile App. Whether the phone is on WiFi or not, the T-Mobile App spits out an error message at login like "Sorry, something went wrong." But when I disconnect the VPN, the app works.

Never had that problem accessing my previous provider app on VPN. Why with T-Mobile?