Why do two different Chat sessions thread into the same transcript or ticket?
When a customer closes a live Chat with your agent, the chat session stays active in the background for some time while they continue to browse your website. This feature is designed so customers can pick up where they left off in case they have further questions.
If a customer opens the chat widget again to start another chat while that previous session is still active in the background, that "new chat" will count as the same session and thread into the same ticket.
The time it takes the chat session to truly end depends on the situation. To learn when Chat a session ends, see the article: When do chats time out?