Hm, must depend on the Shield, then. I tested OD Guards one more time now with the same result: An OD 6 HP Guard attacks through Gravity Shield.
Maybe, both shield and HP changes come into effect at the same time?
Maybe different status effects have different priority:
1. Accel|Overdrive
2. Shield Effect
3. Poison/Purify counter
It is looking like something is going on. (I wish we still had an "ask questions answered by the code" thread like Xenocidius had)
I know how I would have coded it: Categorize effects and then give them priority by category(example in Submarine's post).
1. On Attack abilities (acceleration)
2. On Attack shield (all Shield effects I remember including DR)
3. On Attack status effects (poison/purify)
4. On Damage abilities (vampire)
5. On Damage shield (some kind of scaling retribution shield?)
6. On Damage status effects