Sylas Omnistone PTA does not work

Playing sylas if you receive PTA from omnistone you are unable to gain stacks when auto attacking with his passive active.

1. Take Sylas and choose Omnistone
2. Roll PTA
3. Gain passive for next auto attack
4. Auto attack a champion

Expected: PTA begins process of stacking Result: PTA does not begin stacking unless a normal auto attack

I don't know if this interacts the same way if you take PTA as a regular rune.