That's nothing to worry about. It's how mobile Chrome now works.
It groups similar sites in its own tab now.
As they opened the ACP from the website, it created a "tab" (or better described as a group) of two windows. When they press the + new window button, they can see this as a whole group and swap between the two (forum and ACP), close one to make it one or the other, or close both at the same time.