Not necessarily a bug

Prior to Miss Fortune's minor rework, her Double Up applied on-hit effects and said so. Current Double Up does not say it applies on-hit effects.

In my last game, I had red buff, used double up on a minion and the bounce hit another minion. The second minion, who I had not attacked yet, was then afflicted with the burn from red buff.

Basically, I want confirm whether Double Up is supposed to apply on-hits. If it does, it needs specified in the tool tip. If it does not, then red buff should not have been applied and we have a bug.