iMessage automatically goes to read if it doesn't work correctly. Partner got a new phone number when switching providers. She has a Samsung phone, and kept it. The number they gave her came from an iPhone. All messages sent to her from iPhones were marked read on their end and never get delivered. All messages sent to iPhones appear to send, but don't arrive. All texts from Android to Android still worked fine so we didn't realize immediately as I had an android phone as well.

The phone number that isn't owned by Apple is being routed to them even though they should have no part in the process. Apple's advised solution is to acquire an iPhone to disable iMessages. Thankfully they have a website where you can remove a number from their service, but it is not intuitive to go to AT&T/Verizon/Spectrum/whoever and purchase a cellular plan and then have to reach out to a 3rd party company to shut down their invasive services on products they down own.