This has been happening to me for years on a 6S, and still happening on a brand new XS. I see posts dating back to 2016 on this, yet still no fix and silence from Apple. I understand that this isn’t the most annoying bug to ever exist, but it’s makes it clear that iMessage isn’t fully polished.
For the record I deal with bugs in iMessage & texting in general that are much worse than this. I flat out just don’t receive some messages. This seems to happen mainly when a traditional text is involved, either one-on-on or as part of a group text of both iMessage and SMS users. Worst part of this is it prevents me from seeing any messages, not only the SMS messages not coming in. When I did receive mixed group texts they’d come in as a cluster of separate conversations threads, making it impossible to follow. This wasn’t fixed with the change to Apple’s latest and greatest iPhone. These bugs are very annoying and something I’d consider unacceptable. Once again, how long these issues have persisted is disappointing. Apple has the resources to put people in place to fix these things but for some reason doesn’t. Not sure how long they think they’ll get away with selling $1k+ phones that don’t perform one of the most common functions they’re used for as expected. I miss the perfectionist that was Steve Jobs.