As far as I remember the Heimdall saves u when u have 1% health. It was working till last month (before the update) now it seems to be not working as expected. Heimdall synergy is triggering when the attacker is about to die but the health is more than 1%.
I observed this in EQ, ACT 7 till now.
As you can see for CMM it triggered at 20% and for Venom it triggered at ~10% health.
PS : Please ignore the gameplay
https://youtu.be/jwEpd9lTpl4
https://youtu.be/kS5JUnx6EcA