Sailfish MMS on T-Mobile US has been a long-running issue:

https://talk.maemo.org/showthread.php?t=99925

It's generally just an annoyance, but my experience is that MMSs become inaccessible if they're not downloaded in time. I've missed a few messages that way.

I've learned to live with it (we have bigger issues coming with the eventual US carrier switchovers to VoLTE). But yes, it would be nice if there were a solution that didn't involve manually turning off wifi when wanting to view an MMS.