On version 11.8, Ctrl + Tab opens the next chat of the list in a new window; on the other hand, Ctrl + Shift + Tab correctly moves the active chat to the previous one of the list.
I think the expected behavior, much like any program with tabs support, would be:

• Ctrl + Shift + Tab moves to the previous tab of the list (working correctly)
• Ctrl + Tab moves to the next tab of the list (unexpected behavior: opens a new window)

where the first tab is the chat at the top of the list, and the last tab is the chat at the bottom.