Federico Alzamora Previtali

Negative volume cells is the result of collapsed cells in a dynamic mesh with excessive boundary zone movement within a time step. Looking at your figures 1 and 2, your piston appears to move several times its width in a single time step. Ideally, you would want this movement to be less than a cell width (0.15mm) for any given time step to allow the solver to remesh.

I would suggest reducing the time step by a couple of order of magnitudes. You may also set the Remeshing interval to 1.