It is unclear if this is specifically related to the iPhone 12, but a good number of people who are reporting these issues are iPhone 12 owners. According to one user, “But I just upgraded to an iPhone 12 (coming from an iPhone 11 Max Pro), and I am in a group message with my kids. One of my kids is an Android user. Last night he came up to me and asked me why I didn’t reply to his two text messages he sent to the group. I checked my iPhone, and I didn’t receive any text messages from him in the group chat. He then showed me his phone, and sure enough, he had sent them to the group.”
Some people are noting that these issues seem to involve chats that might have Android users, but we’re not sure how that plays a role. If it is an SMS, it should essentially be platform agnostic, and missing text messages might have to do with your carrier. It is possible that there could be some kind of iCloud syncing issue as well, but that remains to be seen.
It is also possible that maybe carriers need to update their firmware to support the new iPhones, if they haven’t done so already, but either way, it’s something you’ll want to take note of when you get your new iPhone.