Exit callback on particle triggers is not working.

Repro steps:
1. Open Bug_RadiusScale scene in 5.4.0b14
2. Press play and observe particles going to red upon entering the sphere and green upon exiting green.
3. Open scene in trunk (5.5.0a1) and press Play.
4. Observe how particles are no longer turning green upon exiting the sphere.

Expected outcome: particles to turn green (i.e. exit callback to kick in)

Notes:
- regression, works in 5.4.0b14, broken on trunk
- setting Exit to kill instead of callback kills the particles at start.