Being a SHOU form likely isn't the cause; the SHOU forms still point to SPEL forms, which then point to MGEF forms. I'll investigate and compare to a known functional poison MGEF/SPEL when I can.
EDIT: I think that the Cure Poison effect may not work on taper-off effects (like fire damage). The assigned damage is over two seconds, but has a taper effect of four seconds. The taper effect works //after// the effect expires.
Basically, the Cure Poison doesn't detect any poison effects to remove and does nothing. Try gulping down a Cure potion within the two seconds that the effect is applied to test this.