On lvl 1 thrall has 3 talents: blue, red and green lightning.
Green lightning leaves stacks to collect with auto attack.
It works well untill lvl 16, when thrall can pick talent which transforms his 3rd attack after using winds fury into triple hit, that deals 225% (75%x3) his attack damage.
That triple hit doesn’t procc green lightning stack.
It has been in game for at least 4 years and now as blizzard seems to be fixing old buggs there’s chance to fix this one.
It is really annoying and cost me quite a few kills and probably killed me few times by preventing healing I should have received