Maybe Moridin also figured out then, how to Heal burning out via the TP, and this caused the difference in the power drop (also madness can be cured with the TP, after all).

Or who do you think Healed Cyndane in this scenario? We know that Cyndane was already mindtrapped, before Moghedien became mindtrapped in ACoS, ch.25:
"Her [Moghedien's] eyes fixed on the small cage of gold wire and blood-red crystal that he [Moridin] drew out dangling on a cord. She thought vaguely that he tucked another just like it back in, but she had eyes only for her own."

Only channelers can be mindtrapped, which seems to indicate that you can't do it with burned out or stilled persons. Thus Moghedien didn't Heal Cyndane. Graendal doesn't know Cyndane before PoD. Dunno if Semirhage is ruled out as well.

In any case, maybe that Nynaeve figured out the method of Healing stilled (or burned out) channelers was even what Moridin meant with his comment to Moghedien in ACoS, ch.25: "If you had not managed to do some good by accident, you would be dead now."

We know at least that Moridin is very aware of this form of Healing being severed from PoD, ch.2:
"These barbarous rustics offered too many surprises. A way to Heal being severed, however imperfectly. That was impossible! Except that they had done it."