Just realized this is happening on 6.7 still, and it seems to be the same problem from back then.
I've noticed that the focus tab in a stack will change to what seems to be the first tab in order that wasn't active.
By which I mean, say a tab stack has A, B, C, D, E tabs and A & B were active but C, D E were already hibernated. While on non-stacked tab, the displayed focus tab for the stack will be A because it was the last tab I was on before a I moved out of the stack. But when I manually hibernate all background tabs, the displayed focus tab will change to C.
Conversely, if all the tabs in a stack are active, it will always make tab A the displayed tab when hibernated, even if I left the stack with D as the most recent tab.
Shouldn't the functionality be that a tab stack hibernates and leaves the displayed focused tab as the last one you were on?