Hello Santiago,
Sorry for the delay to answer, but with the release of v8p2 branch, I was very busy.
Many thanks for raising this real bug with respect to the theory.
Good to read that you found a fix that enables you to continue to work. It is not totally generic, I will try to find a smarter fix.
Anyway, FYI, just before the creation of v8p2 branch, I added the possibility for the keyword COEFFICIENT FOR DIFFUSION OF TRACERS to be an array not a single value (i.e. one value per tracer) like in 3D. That will enable to give a 0 value to tracers you do not want to diffuse. For advection, I had a quick look but no advection for tracers is not allowed at the moment, a modification may be needed to do it correctly.
I will let you know if I have found a smart fix.
Thanks for your feedback using MICROPOL model,
Chi-Tuan