Hi Newp.
It is intentional. The plugins "turns things on" by activating the animation, but not the processing, to avoid accidentally burning up things stored in campfires, etc. It also reduces the server load, as the animation add a lot less load than processing. Animation is one time cost, processing is ongoing every tick.
There is an option to protect stove (campfires, furnaces, etc.) with the options, so if they are on (or off) and have stuff in them, the plugin ignores them. Useful if you don't activate the insta-smelt option.
-Mal