When I add a tab using the plus icon, it replaces the currently highlighted tab. If it's a tab that can't be closed, the new tab doesn't open.
I can not reproduce the described behavior on 2.9.1. What system are you running - Console or ETCnomad ? Mac / PC ?
I was running an Ion as the primary and my Windows 10 laptop as a client. The bug occurred on my laptop. I am no longer working there. I now have my laptop running Eos offline and I can't reproduce it either.