Just discovered this. Apparently a bug in software. When same guest books two separate reservations, the messaging posted to the first reservation was pulled into the second reservation.
I've not seen this and just had a guest arrival that stayed previously and a booking by another that stayed previously. Just fyi.
This has been the case since I started hosting three years ago. There is only one message thread per guest. So if one guest books multiple stays then only the last reservation confirmed will appear visible on the message details. It's a lousy system that's for sure.
The one downfall is that if you have a guest who makes multiple reservations at once (before the previous reservation is completed), you only see one confirmed reservation in the message thread. For example: you initially have no idea that the guest is arriving also on September 12th because the message thread lists October 1st. It can be confusing if not looking directly at your calendar.