I'm not sure if this qualifies as a bug because I don't know anything regarding the amounts required for making opium.
That said, I harvested a few Poppy flowers at some point within the past few days and got one (1) Poppy Pod. I dried it and ground it in a quern and received .07 Poppy Grist.
I left the grist boiling in a cauldron while I was doing other things and came back to a non-boiling cauldron with enough water to still be graphically full. I checked the inventory and the .07 Poppy Grist was at 100% (I'm not sure if that percentage display is a custom client feature but I'm using Amber's Client in case that makes a difference, although I can't imagine how it would) but there was no fuel left. I lit the cauldron and the instant it started to boil, it consumed enough water to change graphically. I filled it with water from a nearby barrel, and then immediately opened the cauldron's inventory which turned out to be completely empty.
I feel like there's a decent chance that this isn't a bug and that I just didn't have enough grist to make even a single serving of Opium, but I think it might be worth reporting since, again, I don't have any info on what kind of returns should be expected.