Preprocessor simplification incorrectly treats foward-looking equations.
The file simple_impinf.mod contains the line
mu_x(+1) = 0;
Starting in 4.6, the preprocessor simplifies constant variables. The problem is that it interprets the above equation to imply
mu_x=0 and drops the variable from the
lead_lag_incidence. But something that is 0 in expectations is not necessarily 0 in all states of the world.