The bug
When used on a campfire block, /replaceitem says "Block at Pos(x,y,z) is not a container" and does nothing
How to reproduce
/setblock ~ ~ ~ campfire
/replaceitem block ~ ~ ~ slot.container 0 porkchopExpected result
Porkchop is placed on campfire, just like if you right-clicked on it
Actual result
Error
The "Error" is an error message: "Block at Pos(x, y, z) is not a container".
The /replaceitem command only works with containers. This error occurs because a campfire is not a container. The items cooking on it are not stored as an inventory, but as discrete properties (called Item1 etc.).
I checked the wiki and there's no suggestion that a campfire is a kind of container on either the Campfire article or the Inventory article. Why do you expect this command to work? Does it work this way in Java? If not, then I don't see any reason to believe this is a bug.