In theory, maybe the null needs to be flipped 180 on the Z axis.

I had this problem as well trying to angle an exhaust point down. Pulled my hair for 2 hrs trying to figure that out. I figured it was facing back, so maybe I rotated 180 on the wrong axis, I tried them all, screaming at my monitor at each fail. Finally I gave up and customized the particle. I did manage to customize the angle on Flair points but I remember it was a real pain in the ass.

If anyone has a fix, I am all ears.

What will the Ability be doing, maybe we can find a work around.

EDIT:

After a clean fresh look at rotating a null, I may have found the problem, not tested though.
Maybe convertXSI don't like negatives values on rotating Nulls. If I make a new null and rotate it down 30, the value is actually -30. I can get the same position by rotating clockwise to 330 (360 - the down angle you want). Try that, I have to leave now or I would test for you.