Forge version: 44.1.23
Twilight Forest version: 4.2.1616
Link to crash log:

Steps to reproduce:

1. Get the Lich a good distance from one of the clones (don't know exactly how far it has to be, hard to test since he teleports all over the place). The distance is great enough that this won't happen in the Lich Tower arena, but it could easily happen if the Lich teleports out of the room or if you fight it somewhere else.
2. Break the Lich's last shield, moving it into phase 2

What I expected to happen:
The clone would vanish, as the clones normally do so when the Lich moves into phase 2
What happened instead:
The clone stuck around and would continue to attack if approached. Interestingly enough, it did disappear when I killed the Lich during a test.