Steps to reproduce/explanation:

a player being given damage from an entity causes knockback from the direction the damage was coming from:

/damage @p 3 minecraft:fly_into_wall by @e[sort=nearest,limit=1,type=!player] 

however, if the location is signalled, no damage is given:

/damage @p 3 minecraft:fly_into_wall at ~1 ~ ~

This behaviour hasn't really been defined, but I think it would make sense for the 'at' signifier to declare the location the damage was dealt from.