Grave B can whiff when the opponent is very close to the opponent (e.g. walking towards Grave point blank)
^ works on the entire cornered cast except for Grave and Midori. Works mid screen for Rook and Dragon Midori.
Since I have no access to FS hitboxes, I can only guess that it's caused by the fact that projectiles don't use the "long hitbox on the first few active frames" trick which exists in SF3/SF4/SF5 to make them more consistent, but I may be wrong: