The participants will view the chat message from the time they joined the Webex Meetings. Any previous chat messages that took place before they had joined will not be displayed and this is by design.

For Example:
The meeting started at 2 PM, and Participant A joined the meeting at 2:10 PM. The chat conversation that took place in the meeting between 2 PM to 2:10 PM will not be displayed to Participant A.