And this is why items aren't allowed in tournaments.

A more detailed explanation:

There are weird ceiling collisions in the underside of Vanilla Brawl's FD, it stalls your double jumping momentum and is generally a bad place to be. While Melee FD's collisions act like walls, it means characters like Toon Link and C. Falcon can wall jump from them and momentum based Up-Bs like Ness's or Fox's can ride walls to the ledge.

Also, if you want Vanilla Brawl's FD back, delete the "STGFINAL.pac" file from the stage folder of the Project M files.