It seems to be the fault of the density unit. I’m not familiar with the imperial system but I know that density units become particularly tricky when a consistent imperial unit system is required in FEA. It seems to be the same problem as here: Centrifugal Loadings - Imperial Units Error? - V1.4.1+
and, for whatever reason, dividing the density by 386.1 gives reasonable results.
Old thread about those units in PrePoMax: Density to Mass Conversion (Modal Analysis)