>In a Double Battle, Leech Seed will only heal the Pokémon that is in the same position as the Pokémon that originally used Leech Seed was in. If no Pokémon is in that position (for example, if it fainted and could not be replaced), Leech Seed will not take any health from the seeded Pokémon.
I believe this is the case. As Bellossom was automatically centered, there was no one in the position it previously was in and therefore your Jellicent stopped getting seeded. It might be different because this was under the Gen IV header and it only refers to Double Battles, but I doubt it.
Hope I helped. :)