Raw mineral bonus (granite 0.2; sandstone:0.1) is not a "bonus" for the decor value but a multiplication of the base decor value . Can see this for theouthouse where base value is -20 and a built granite outhouse is at -24.
Steps to Reproduce
built a granite outhouse, check the value given by the decor overlay!
Status: Known Issue
We are aware of this issue and have it logged internally but either we do not have a fix ready yet or we are not planning on addressing the issue in the immediate future.