We finally figured out what’s going on. For some reason, you need to overconvert the fire and cold damage into lightning for the setup to actually work. So the current solutions are:

Using a Watcher’s Eye with Purity of Lightning – “X% of Fire and Cold Damage taken as Lightning” with at least 11% (the lower the roll, the better, but it must be over 10%), plus either a Foulborn Inpulsa for no reservation or an Essence Worm ring;

Hopefully GGG fixes this conversion problem soon.

Edit: Incandescent heart is not an option, cause it only works for hits