I checked and in both cases you're pressing B2 when your opponent hit his strike (to do a good slice if he shots near u?), then you release, then you press B1+B2+up ( = short accel) and don't release before hitting the ball.
With the .dmo, everything is alright ; on the BH, it starts with a slice anim but swap to the normal anim as soon as you press the short accel buttons.
I checked in the code and couldn't find anything wrong linked to that kind of sequence.
So I tried to do it by myself within an online match, but it worked also like intended, so it's quite mysterious.
Could you tell me the # of your FH & BH ? (in the Character Sheet config screen)