This also affected one of my kids yesterday. They texted another iPhone user though. The originating text was converted to international number with the '+' sign. Both are iPhone users. One is Verizon the other ATT. The problem was on the ATT end user's phone. They resolved the issue by deleting the thread with the bad contact number, closing the messaging app, and restarting the phone. Everything was normal after that.
EDIT: I just remembered the funny part of all of this. My kid was trying to figure out who was hijacking their conversation when they contacted me. I asked them to send me the phone number disguised so that it isn't picked up as a phone number in the message app and I would check it out. A minute later I get a phone number that is obviously their own phone number. I pointed that out to them and had a good laugh. At that point we started blaming it on the app, not on some message international hijacker. Pretty funny that neither person in their conversation recognized the phone number.
One of them does but the other doesn't use WiFi messaging in iMessage. I don't have a clear understanding of why they don't. I asked the same question. It evidently has something to do with one iMessage user enabling WiFi messaging and the other prohibiting it so you end up green bubbled as if you're dealing with an android user.
iMessage is super finnicky, particularly if either end of a conversation doesn't have their WiFi/cellular settings configured just so and everything else lines up. I'm quite glad people are picking up Signal more and more.
I normally hate when congress gets involved, as it cements us at "the current generation of tech" for a long time (Think what happened with copper telephone lines).
But I too, am tired of not being able to interact with Android users in a smooth manner. It's obnoxious, even for Apple users, and it's Apple's fault. The world runs on Android, despite what Apple claims.
I've had another bug. I use an iPhone with one physical and one eSIM. I swap out the physical periodically, and any SMS message threads I had with Android contacts using the physical sim become broken. Sent messages will stall out and never send.
I am required to delete the contact, delete the message thread, restart the phone, create a new message thread, then re-add the contact via the thread. This works ~50% of the time. I have to delete the contact and the thread because both of them store some info about the SIM being used and it appears to get cached somewhere preventing me from sending more messages with the new SIM.
My assumption is the issue originates from the profile that is created for physical SIM. Anyway I'm never upgrading my iPhone because I'm not gonna give up a physical SIM slot.
EDIT: I just remembered the funny part of all of this. My kid was trying to figure out who was hijacking their conversation when they contacted me. I asked them to send me the phone number disguised so that it isn't picked up as a phone number in the message app and I would check it out. A minute later I get a phone number that is obviously their own phone number. I pointed that out to them and had a good laugh. At that point we started blaming it on the app, not on some message international hijacker. Pretty funny that neither person in their conversation recognized the phone number.