By default, it doesn’t seem your heroes try to avoid fire zones when asked to move. If you ask them to sprint, it seems there’s no way to modify their foot path or force them to run through waypoints for instance. They’re likely to run directly towards zones with fire (which is nonsense). And if they die by doing so, the game tends to freeze. Annoying.
The heroes, if not in fire already, should avoid running through zones with fire at all cost, by default, or at least there should be some way to ask them not to run accross fire. My 2 cents.