I don't think Saded Devour will double proc Jax's W and R

I could be wrong because I don't have a PBE account, but I'll post my reasoning regardless

They don't behave like standard on hit affects, because they aren't; they are single target spells and always have been.

Afaik, there are big differences between autoattack modifiers or on hits in this game. There are standard on hit affects, single target spells, physical on hits that proc lifesteal, and even rare magic damage on hits that proc lifesteal (Nidalee's Q)

On hit effects in this game do not proc life-steal or spell-vamp. Something like Warwick's passive or irelia's W. Jax's W and R are classified as single target spells for all intents and purposes. Both of them proc spellvamp at a 100% value and get the full 35% slow from Rylai's. They behave exactly as Akali's mark of the assassin does.

To compare, something like Aatrox's 3rd hit does not proc spellvamp or rylai's slow, instead it behaves like a normal attack.

If someone with a PBE account could confirm/deny my suspicions with proof, however, that'd be awesome.