I have noticed this recently too, often while the full conversation is visible on the AirBNB message dialogue page when viewed via a web-browser, when viewed on the mobile app, various messages seem to randomly not show in the discussion for some reason.
I had a similar experience this past week. I responded to a booking request on my laptop one hour after receiving it. It showed as sent. But it never reflected on my mobile app and the guest did not receive it either. When I went back to my laptop the following morning it had disappeared there as well.
From now on I will double check that the messages reflect both on the app and the laptop.