I feel so seen! This is happening to me and my wife as well!
we both upgraded on 10/23/20 to the iPhone 12 Pro and have Verizon and we are both missing random SMS/MMS messages.
I’ve contacted Verizon and Apple and both pointing fingers at each other and no one can give me any answers.
I've tried resetting the phone, resetting network, hard boot, removing SIM card, deleting thread of SMS/MMS messages.
I cannot find any rhyme or reason to which messages I’m missing.
I’ll get a few and my wife will miss those and then I’ll miss a few and she’ll get those and then we both will get a few and miss a few.
So frustrating!