Is this a bug?

So I believe I learned today that conqueror does not stack with garen's spin. Not even 1 stack for initially hitting with it. Is this a bug, because I could've swore that Darius gets stacks just from his bleed being on you. Similar to how you get stacks of black cleaver while spinning/Darius bleeds, why does this not apply to Garen's spin and Conqueror?