There's a glitch in Triple Battles where automatic center apparently makes Leech Seed stop working on Pokemon.
On turn 7, Leech Seed works, but after auto-center, it doesn't: http://replay.pokemonshowdown.com/randomtriplesbattle-400226321
Leech seed does sap into the users slot rather than into the user.
However it shouldn't auto-centre as the pokemon are adjacent to each other.
I just tested this on-cart, 3v3 free battle.
9W6G-WWWW-WW4F-5GJN
The final turn is manual shifting to the centre, not automatic.