The bug

The "fatal" damage filter triggers when the entity receives fatal damage, whether that be from a player, or a lightning strike, falling into the void, etc. However, it does not trigger when the entity is killed using the /kill command.

Steps to reproduce:

1. Enter the provided world, with the example behavior pack and resource pack.
2. Summon the test entity (test:sample_entity) using either a /summon command or the provided command block.
3. Kill it with your hands. Notice how it shrinks.
4. Repeat step two, but kill it using /kill @e[type=test:sample_entity]. Notice how it does not shrink.

The damage sensor on this entity sets a component group that sets its scale to 0.2 to demonstrate that the trigger succeeded.