It could just be very seldom that such a thing happens, so that most Forsaken don't know about it (if the Power drop is related to Cyndane's weaker body). After all, it seems the Forsaken don't speak very much about transmigration (they don't talk that much with each other anyway), not even mentioning the old names of the resurrected Forsaken, or the transmigrated Forsaken explicitly telling, who they were. It's probably against Forsaken etiquette.

Especially if it were really relevant for the plot that Moiraine becomes stronger in the OP, it would IMO seem like such a made-up, cheap deus ex machina explanation that RJ got the idea, that when two channelers fall through a melting doorway ter'angreal and simultaneously use an angreal this obviously leads to one of those channelers lose e.g. 10 % amount of her OP strength, which the other channeler gets somehow (and Moiraine was apparently simply lucky that she was the one who got the strength). As said, the OP strength is tied to the body of the channeler. Seriously doubt that such a natural ability could partly wander to someone else's body.

Don't think this theory can be correct, but we will see.