So it happens only when switching to death animation by this event?
You didn't expand it on the screenshot, so maybe there is something wrong inside of it. Or somewhere else in your code...
Rename Fade behavior on the boss sprite to a unique name like "FadeDeath" and search for this name in all event sheets.
If nothing helps, as a workaround you can add an event that will disable Fade behavior on frame 0 of death animation.