actually, not sure to be honest.
most other immobilizers prevent scattering from kb/fear.
I usually compensate by using a hold before propel to prevent KB.
Propel itself is an odd power as even if it misses the aoe KB effect can still occur, doing no damage whatsoever.
grav also seems to do very little damage which is why is has multiple power interactions.
Containment
overpower
impact
You can set up some nifty combos using the interactions, but they are slightly random.
it's possible something went screwy, or something was overlooked when the interactions were put into effect OR
it might be working as intended and we simply don't know why.