I just tried again on another server, it turns out it was caused by a repeating command block giving jump boost to players who are in the air (to cancel fall damage).
/execute @a ~ ~ ~ detect ~ ~-1 ~ air 0 effect @p 8 1 255 true
I still think it is a bug though since the velocity the projectile gets isn't the velocity of the player at the moment the snowball is thrown.
I'll close this and open another one unless it is not a bug.
Added the screeenshot with your command running.Also added 16w07b.
I know this is a year old issue, but I'm just here to confirm that it still is a bug on the following versions: 1.8.9, 16w06a, and 16w07a.
Added 16w07a
I just tried again on another server, it turns out it was caused by a repeating command block giving jump boost to players who are in the air (to cancel fall damage).
I still think it is a bug though since the velocity the projectile gets isn't the velocity of the player at the moment the snowball is thrown.
I'll close this and open another one unless it is not a bug.