Hello, first off nice game but i've hit what i think is a bug...Every time a forest catches fire my survivors will always go and collect the ashes even IF i de-flag the ashes for hauling, the game just re-flags them to be picked up.
Check your autopickup settings. Hit P or hit the autopickup button in the top right. Tell them not to automaticly collect the ashes. That should keep them from getting them! happy you are enjoying the game otherwise :) Do let me know if you find anything else or have more questions.