Oh there’s actually a function available in BPs called “Launch” which applies an instantaneous force in a direction (either additively, or replacing velocity components) and sets the movement mode to Falling.
If you use your Branch Point’s event in your AnimBP to hit a Cast node (JumpAttackLaunchBranch > Try Get Pawn Owner > CastTo MyCharacter > JumpAttackLaunchEvent) you can call a custom event on your Character BP which Launches the character based on his current forward vector; e.g. Get Actor Forward Vector > Vector * Float (e.g. 800) > Vector + Vector (to add like 200 Z or something so the character arcs through the air a bit) > Launch, override XY, override Z)
That’s how I do it for my Skydrop attack, anyway (where the player plummets downward and then hits the ground in a landing).
EDIT: To answer your specific question, telling the animgraph “he’s not grounded” won’t achieve anything. The capsule will constantly report that it is IS in fact grounded unless it’s actually not touching a surface it can stand on.
And being airborne, the Z values of root motion WILL be applied, but remember that gravity will still affect the capsule, so it won’t work like the animation suggests. If you really want to have total anim control over the aerial position of the actor, you need to either make him airborne and disable gravity or, alternately, set the movement mode to flying. That way, only the root motion controls where the capsule is.
Personally I think it’s easier (since it’s just a jump and not something like a complex aerial maneuver) to just use the Launch node and not worry about root motion for that point of the anim. You’ll have to tweak the XY and Z values of the aforementioned Launch a bit to get it to feel how you want, but in the end it will work better.