Rageblade doesn't work on Diana's passive

I've been playing a lot of Diana lately and have been trying out a few builds in practice mode. I've noticed that when you have max rageblade stacks (which makes every other hit trigger on-hit effects twice) her passive doesnt trigger on every other attacks as opposed to the third. It works for other 'third attack based skills" likes vayne and gnar's w and yi's passive, so why not Diana's? I feel like it completely removes the viability of Rageblade on her, which would be an insane split push style to play her.

Intended?