I actually have no idea but could be the contour range? Looks like most of values are located between 0~5e-7. So you can adjust the value range to check.
Also, solid time step is much smaller than fluid time step, when set des_explicitly_coupled = True, we do a coupling to cast fluid info to particle at the beginning of fluid time step, then during this fluid time step, we assume fluid is stationary and perform solid iteration until solid time can march fluid time. It is not as accurate as continuum coupling (recompute drag at each solid time step). Maybe this also affects the drag.