Originally posted by us271934-Слава Україн:

Plenty of smoke exit points. So far putting a barrier between the hearth and campfire is keeping the campfire lit.
https://steamcommunity.com/sharedfiles/filedetails/?id=2799668574

Best way I can explain all this(and some are sorta covering it) is to think of the smoke from a fire as a block of stone. When the fire makes 1 block of smoke every few seconds, it has to move up and away. If it does not, then the game turns off the fire.

If the blocks move but start to accumulate to the point where they can create a traffic jam of blocks that end up backing up to where the fire is, the same stopping of the fire will occur.

The on/off, is due to the blocks eventually dissipating that are older and a new block shifting into its previous place and being checked by the system(usually) to ask the fire to turn on or off.

Building in certain ways then can "guide" the movement of the blocks of smoke. Other wise, they'll pick a direction(or alternate/randomize). This may be why your "barrier" allowed for the smoke to be guided in a different direction.

Looking at your picture, it is very obvious that the roof of that structure is very low for a hearth and why you have to have such ventilation more open and somewhat guided. You can clearly see the smoke blocks accumulating too much and low even with that. Personally I would redesign it to have a much higher roofing with ventilation perhaps.