Forestry_1.12.2-5.8.2.422
Forge-1.12.2-14.23.5.2852

Humus doesnt seem to 'degrade' when a sapling grows on it so it doesnt turn into sand no matter the config

I'm curious how you tested this, because I was able to get humus to degrade using Forestry build 422 and Forge build 2854:

However, by my calculations, the median time should be about 40 minutes to degrade one step, though there is a 2.78% chance of it taking more than 3.47 hours. Also, if you have humus directly below an isolated tree like that, afaik it will never degrade, even with the PR auto-linked above.