I'll bet this is a step resolution problem.  In the real world, there are
some unimaginable number of "steps" per unit time, and things are happening in
all of them- therefore, a nice overlap instead of seeing room sized quantum
effects.
    Your artifact is undoubtedly due to your quantum of time - the unit frame.
Perhaps you could calculate the interpolation frames and where those smoke
particles would be and what their diameters would be for the in-between stages.

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip

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