I was thinking this would be server side, but then that would be ignoring the fact that two distinct numbers would not be able to become "merged" with the way SMS routing works. I only have experience working with carrier resellers (i.e. Bandwidth, Voyant, thinQ, Spectrum) and the Twilio API, but think I can weigh in with my professional opinion. find it hard to justify that this is a bug with the exchange between carriers or endpoints.

What has likely happened is that the sender of the message's app crashed quietly in the background while the message was being typed. The way the sentence is structured looks like it was composed with autocomplete - when you just keep hitting the first recommended word that pops up. Somehow autocomplete was submitting the suggested word into the SMS table running on the senders phone, while the sender is still seeing what they are typing on their screen. Once they hit send, their SMS app commits the text on screen to memory, but the "delta" full of autocomplete words was transmitted to the carrier, then to the recipient.

If this was an end-to-end encrypted conversation, the message would not have been sent.

I think the sender might want to run some sort of AV on their phone, as the message was technically modified without consent of the sender, and could point to some shady business happening on their phone.