Well shit. Sounds like Apple has an issue clearing cache. In the hopes of actually solving your issue, I hate to suggest this, but try reconnecting your number

Well shit. Sounds like Apple has an issue clearing cache. In the hopes of actually solving your issue, I hate to suggest this, but try reconnecting your number to imessage (and make sure you get any new imessage texts), then properly turn off imessage from that device. The defacto unplug it plug it back in.