Can't even replicate the bug, but in complete description:

I had 2 coals, 3 furnaces, 16 dark oak logs to burn into charcoal and 4 porkshops with another 4 logs as fuel. Can't recall what I did in the meantime, but I never left the ovens, but after some time I checked the furnaces and I only had 8 charcoals, the porks but the other charcoals supposedly dissapeared.