When adding a third person to the group chat call, the third person is not able to see the previous chat of the group. The person is only able to see the chat messages that will be exchanged during their existence in the group chat. So when you remove this person from the chat afterwards and readd them a second time by calling them (maybe the next day), they still only see the chat massages that were exchanged during the time when they were in the conference call the first time.