Hey everyone, I hope this helps:
I struggled with this issue for awhile and spent countless hours on the phone with AT&T and Apple. There are plenty of threads online about this issue with no solutions. I noticed this back in the day when I received my launch 7 Plus, I would get group texts or picture texts (MMS, not iMessage) and then randomly I would just stop receiving it. Sometimes rebooting the phone or switching from LTE to 4G/3G would pull in the messages I didn't receive but not always. I had to turn off MMS, reboot, enable and reboot again to temporarily get it working again (all undelivered texts would be lost). AT&T swapped my sim, changed my plan temporarily and back, escalated the issue and found nothing wrong...blamed the software on the phone. Apple had me reset network settings, then reset and restore from iCloud since I always used iTunes backup/restore and I had the same issue. Apple had a senior manager communicate with me and I sent logs to the software engineers, in the end they blamed the network. It was basically AT&T and Apple pointing the finger at each other. My last straw was missing family Halloween pictures and event texts. At the time, since i was troubleshooting for about 2 months with Apple, I was over the return period and they did not let me return the phone :/..I sold it at a loss and switched to Android for a few years because I didn't have any messaging issues..hello Pixel!
Last year when I received my 11 Pro I would randomly have this issue, at least once a month then it started happening more often. I went through the same troubleshooting steps, AT&T swapped my sim, escalated to a network engineer and they found no network issues. Apple suggested resetting the network settings, reset settings and then eventually the dreaded
RESET THE PHONE AND START AS NEW. I really didnt want to do that because the main feature I liked was backing up/restoring from an iTunes encrypted backup and get your iphone back up and running within minutes without the need for entering passwords. Well it kept happening and I eventually sucked it up and reset as new. It took a few days to get everything back in order and I'm 4 months without any text problems.
TLDR: If you set up your 12 from a backup, reset your phone and start as new.
Tip: Screenshot your home screens and email them to yourself before resetting
My educated guess of the issue that Apple won't find/fix: I think backups pull firmware or settings from the previous modem chipsets and do not clear with the new phone. I noticed my issue with the switch to Intel modems (7 series on AT&T and Tmobile versions). I had iPhone 5, 6 Plus, 6S Plus and SE 1st gen without any network/text issues from backups. Now they switched from Intel (iPhones 7 through 11) to Qualcomm for the 5G modems.