My best guess would be that the force is being applied and then being counter-acted immediately by the Run. So, what I would do is create an option to lerp the player's movement like below for a short duration 0.2s to 0.4s, when you add the force :D. That should hopefully allow both forces to to apply but if you find the Y force is still much stronger one thing I've done when experimenting with adding a Dash is multipliying the Y velocity at the same time the run lerp runs out to balance the X and Y forces out. (Similar to what we do for the Jump Cut). Hope this helps!