To quote myself< "Could the hearth's smoke 'blob' be randomly interfering with the campfire's blob? In effect trying to stuff 2 blobs into a space that can hold only one?"

The smoke blobs can be seen as ball shaped clouds. Sometimes there were gaps between them like this and other times they were bunched up.

https://steamcommunity.com/sharedfiles/filedetails/?id=2799815651

Added a roof tile - no more smoke intrusion from below.

https://steamcommunity.com/sharedfiles/filedetails/?id=2799817916

This one still has me puzzled though:

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