Steps to reproduce:
Quit and relaunch Orion.
Open Orion with the tab group that was last active. For some reason it always chooses to open one specific one of the tab groups instead. Not sure what the logic behind the choice is, but I think it's the last one I created, or the last one I renamed. It is not the first one in the tab group list. (It's the second one.)
Orion, OS version; hardware type:
Version 0.99.123.3-beta (WebKit 6220.127.116.11)
MacBook Pro (Retina, 15-inch, Late 2013)