A message sent from the phone at 8 pm did not appear in Bitrix24. Can that happen?
Rarely, and only if the connection was down at that moment; a connected number delivers every message, with retries, even during Bitrix24 outages.
On a connected number, no. Every message is stored by WASync before it is passed on, and the hand-over to Bitrix24 is retried for hours when Bitrix24 is slow or refuses it; an undeliverable message raises an alert instead of disappearing. Messages typed on the phone are mirrored the same way.
What can lose a message is the connection itself being down at that moment: a QR number whose linked device was removed, or a phone that was offline for a long time. Check the connection's history in Connections around that time; if it shows a disconnection, the message never reached WASync and is only on the phone.