It seems that the chat facility notifies all users that have opted to be available for incoming chats. Selecting only the topic you want to be notified for is being ignored. As below in the screen capture, I only want to be involved in the support chats, however anyone that has enabled the chat can take the chat, as it notifies all logged in users whether that are opted/enabled or not. Ie, all users have the chat option, but not all have the topics enabled.
The chats make use of a difference chat ID as required to separate them.