"Substitute will not alter the execution of Bide, Counter, Disable, Haze, Leech Seed, Super Fang, Transform, or partial trapping moves, nor will it absorb crash, recoil, or recurrent damage". I suspect that this applies to Pokémons that were seeded _before_ they went behind a Substitute. Do you have a source saying that Leech Seed cannot be used on a Pokémon behind a Substitute?