In this game, the fire spreads via raycasts and emitters. If you go into fullscreen and ignite something, the fire doesn’t spread correctly. There are two possible reasons for this.
Why did you even make a forest fire simulator? I hope the player character gets arrested in the game’s ending 
As for the bug, it worked for me properly even when I was in fullscreen
Maybe you play as a fire fighter that is trying to put the fire out on a realistic simulation of a forest fire.
You literally start fires and deliberately ignite the trees
Those are fireworks, not trees. And the green stuff is actually oil if you look in the game editor.
Anyways, the bug is happening for me as well, but only when the fullscreen behavior is set to “Expand”. It works properly when set to any other fullscreen option.
Oh. After replaying, I see it now. Both the bug and the evidence proving that this is not an arsonist simulator like I wrongfully thought and is actually a fireworks starting simulation.
Not related to your problem but I really like the firework animation