Highlight the groups a contact is in?
I think you found a bug.
Within All Contacts,
if you select one contact entry,
hold down ⌥⇧, maintain them, and select a second contact entry
then the highlighted group will stay on.
From there, if you use the keyboard arrows, this highlighting will remain on, but won't be correct.
My analysis is that in this sequence of events the ⌥ key up isn't managed correctly.