Why the Fire Dragon won't die without an event?

Here I have a video of what I meant…

As you can see, the Fire Dragon won’t go down if we don´t use the special event for its death, he just stays alive.

Does anyone know why he does that and how to prevent it?

This may be obvious…but it has to do with the coding of the Fire Dragon! Coming from one whos inspected it already, it will only die to AN event that causes it to…well, DIE! Intelligent Systems made it so it HAS to die to an event.

You know what event it is at least? I managed to kill it using Poison weapons, think not only by poison damage but by normal damage once poisoned.