Did some further testing:
The campfires need to be in the same subchunk as the lag-generating contraption, not just the same chunks. Hope this helps finding the bug in the code.
I can recreate this bug reliably by generating a lot of lag in a single chunk (e.g. with observerclock + redstone). With all fireplaces inside the 'lag-chunk' this bug occures, but with fireplaces in other chunks it does not (see image below)
Also I attached my Test-World for easy reproduction of this bug.
[media][media]
newest version (19w04b) is still affected.
Also this problem only occures in the same subchunk as the lag-generating contraption, in other (sub-)chunks this smoke-duplication does not appear.