It's possible that the plugin might override Game_Action.prototype.evalDamageFormula, which means "c", "d", etc. would never be defined. For some reason I find incredibly irritating (it doesn't let a game dev find errors in their formulae) any damage algorithm errors simply cause the damage to resolve to "0" instead of throwing an error, so it's hard to say without looking at the plugin in question. And I can't reach SRD's site right now.
Viewing post in Bug?: MAT based tech issue
I agree, it's incredibly irritating that instead of showing an error, it just resolves to 0. Unfortunately, I have not much knowledge on RPG Maker MV and the scripts behind them, but I tried opening the plugin in notepad, and I could not find any "damage" related script.
Could it be an Actor ID issue instead? I did find tons of reference to actors in SRD's script.