Here’s a really weird one...
I keep Messages synced between my phone and iPad via iCloud. It’s worked perfectly for ages. Yesterday, I realized that I hadn’t been getting the latest texts, for a thread including my wife and daughter, on my phone. They were coming in on my iPad, as normal.
The iPad, being a wifi-only version, is tied to Messages using my AppleID and iCloud. My phone, is tied to Messages with both my AppleID and phone number. Looking into Settings > Messages > Send & Receive on my phone, I discover that my phone had, at sometime Sunday afternoon, been disconnected from my AppleID. Once I reconnected with AppleID, the sync worked once again.
But, I’m left scratching my head over how the phone got disconnected in the first place? I never get into those settings.