I know I’ve had it work correctly before, but more often than not it fails to do the exile. This time however I got a video of it. Nissa has a 10/3 token stack (toughness <= 5) and there are 8 void gems on the board to meet the 6+ requirement (10 if you count the 2 from the ingest on the card that should be triggering before the void gem check).
If I had to guess it is based on some sort of “base” value after reinforcements are applied but before damage is.
I’ve noticed it both working and not working, so that’s my guess.