The problem is that when the character land, the vertical velocity will be at some positive value, lets say 732, due to gravity, and it is never set to 0. When you later try to jump, you add the jump force (-1000) to the existing velocity. So velocity.y = 732 - 1000 = -268. You now jump with a force of -268 instead of -1000.
To fix this you must either set the vertical velocity to 0 while the character is on floor, or set the vertical velocity to a fixed value when jumping instead of adding it to the current velocity.
velocity.y += delta * GRAVITY
velocity.y = 0
if Input.is_action_just_pressed("ui_up") and is_on_floor():
velocity.y = JUMP_SPEED