I made a small pen with a soul-sand floor, so the resource hogs could dig truffles and farm soul sand
The problem is the pigs are not moving at all, and therefore not digging any new truffles, meaning I have to dig every single truffle before the hog will dig a new one
Is it because of the soul sand?
Is there any way to fix this?

This issue is fixed in c354ea4

Thanks for reporting this, it's a known issue with all resource hogs. They are very interested in the block below them and none of their AI will run once they're on a block they like.