Yes. P9.x parameters are not write-able via GSREGWR since those are communication parameters. It never reported an error, but the values never got written either, so the logic was always "not working".
If you do need to configure specific Comm parameters at startup, use MWX and write to those parameters via Modbus. GSREGWR uses the "block transfer" method, which dis-allows P9.x parameters. If you use Modbus, you are by-passing that GSDrive "block transfer" mechanism.