@lduboeuf Thanks a lot for the suggestions. That's interesting to hear that the nuntium restart command should be the same as rebooting. It happened often enough that I feel confident saying a reboot didn't happen.
Now that I've heard your idea about booting I wonder if the trigger that would cause the issue was reboots all along. I'd be disappointed with myself for not making that connection, but it's definitely possible.
I don't have this issue anymore, it occurred both on a previous phone (Oneplus 5T) and with a different cell provider (not sure which change fixed it), but I'll try swapping my SIM into that broken old phone and see if I can reproduce it just to see.
As ever, thanks so much for your work on MMS and for the help!