Dupes constantly refill critter feeder with tiny amounts of polluted dirt as it evaporates

While using the critter feeder filled with polluted dirt, duplicants constantly refill with tiny amounts, marked as "0" (rounded down from something v small!)

It seems as the polluted dirt evaporates, the dupes have a job constantly added to top it up by these tiny amounts, so they'll refill it easily ten times in quick succession with "0" amounts before wandering off.

My rancher is so busy constantly adding a few grams of polluted dirt at a time he's neglecting his other duties...