It's haunted...by Valheim physics.
I removed the spice rack - no change
I removed a 1m beam on the wall side of the floor cutout around the fireplace (above left) - no change
I removed the beams supporting the spice rack - no change
I then removed two 1/4 floor boards flanking the notch in the stone wall thinking they might be impinging on the smoke requirement space - success! Even though they were optically not hanging over the edge of the wall notch. I replaced them and the haunted physics kept the fire going. I will say that the floor board placement on top of this tower isn't perfect but the boards were visually flush with the notch, not hanging over. They don't look any different now than when the fire was going out. I bet it was a Wraith!
Anyway thanks for the ideas.
https://steamcommunity.com/sharedfiles/filedetails/?id=2799469270

Bonus screenshot confirming the existence of Valheim physics

https://steamcommunity.com/sharedfiles/filedetails/?id=2799469639

Edit: I forgot to say "no mods"