I have the latest version of CC. I have a simple group containing a filled path and a more complex sub-group of objects. I can right-click the whole group and isolate it from the rest of the document by selecting "Isolate Selected Group" from the right-click dropdown, but when I try to further isolate the more complex group inside, the option on the right-click dropdown is greyed out.

Typically I prefer to have the "Double Click to Isolate" deselected from the General Preferences because I often mistakenly enter Isolation Mode when I don't want to be because I've accidentally double-clicked a group. When I turn the option back on, I can double-click to isolate the more complex group once I've already isolated the whole group even though the option on the dropdown is still greyed out.

I've tested the exact same scenario in CS6 on my second computer, and in that version of AI I'm able to isolate the more complex sub-group by both right-click and double-click, so I'm pretty sure this is either a bug or I've got some weird option checkmarked that I don't mean to have checkmarked.