I had a related issue very recently. This was the only discussion I could find on the topic, so I’ll post my experience and solution here for posterity. It’s looking like mms support is around the corner, though, so hopefully this won’t be needed soon.
A friend had shared with me a list from their text app (some samsung note taking app), and his phone sent it as an mms - at least that’s what it appeared to be initially. I didn’t receive it, so he sent it again. I suggested breaking it up, and those didn’t work either. After a simple text to test, I found out I stopped receiving texts entirely. Sending texts was not an issue, and flashing to byzantium did not fix the problem.
Found this discussion after looking it up, and I freed up most of the backed up texts after loading my sim into my backup android - except those two that had been sent initially. After working with purism support, they had me run sudo mmcli -m any --messaging-list-sms
[sudo] password for purism:
/org/freedesktop/ModemManager1/SMS/14 (receiving) /org/freedesktop/ModemManager1/SMS/13 (receiving) /org/freedesktop/ModemManager1/SMS/12 (received)
...
/org/freedesktop/ModemManager1/SMS/1 (received) /org/freedesktop/ModemManager1/SMS/0 (received)
After removing entries 14 and 13 with sudo mmcli -m any --messaging-delete-sms=[index], the problem was resolved. I can receive texts once again!