Hi,
I have tested this issue in the latest version of STAAD.Pro (Select Series 5, 20.07.09.31).For ISMB225_CP section, plastic modulus about V axis is reported as 177406.67 mm3 whereas for ISMB250_CP section, the same is reported as 200913.04 mm3. I have attached the sectional property reports from section wizard for your reference. The values are reported correctly. We suggest to upgrade to the latest version.