CC
HH
FF
HH
CC

C : Chest
H : Hopper
F : Furance

I placed a lot of cobble at the chest above to create smooth stone. After placing lava bucket in both the furnace, the first one will have a bucket but a the bucket in the right furnace disappeared. I can repeat this many time and it still happens. The furnace will have the animation for cooking but the stone don't seems to be cooking.

I tried another configuration

CC
F C
H F
CCH

And the right furnace still cause the bucket to disappear.