Master and Pet are treated as a single entity when counting towards stacking totals.
Meaning; An enemy only touched by Master will still count toward the Pet’s Master’s Bond stacks, in much the same way something only touched by the Pet will still count toward Master’s Sigil Stacks. So you can use AOE tagging to help ‘feed’ your pet’s MB stacks.
I don’t know if that came this patch exactly, because it’s been months since I last touched that trait. But formerly, the pet had to personally touch something for it’s death to count towards Master’s Bond (as though it has been given it’s own independent Sigil.)
Just thought I’d share.