If you look at the variables in your Projectile. In the Details tab, there is more options you can enable. Search for Expose on Spawn for the variables you want to see when you Spawn Actor for the specified blueprint. Then you can set them On Construction, and the Projectile Component will read the updated variables.
Because whats happening is that if you set it the variables after the Projectile has been spawned. The velocity from the default variables have already been applied. So it continues to land in the same place.
Yes you must look in the Variable section. You want to expose your variables, then set them OnConstruction of the Blueprint Projectile. Basically setting the properties before the Projectile Component initializes and shoots your .