Thank you so much for playing! Your criticism is on point and I completely agree. The fire works by averaging heat between adjacent tiles, and burning tiles increase in heat until they are out of fuel. What I should've implemented was ambient heat dissipation. Instead in a last-ditch effort (no pun intended) I put per-frame heat reduction in the trenches (which otherwise are just tiles with no fuel thus cannot burn). Water reduces heat per frame for the tile it's on, but yeah, it's too weak.
Here is an image of the heat spreading. I considered leaving this debug mode in the game haha
Thanks again for playing :)