Maybe this floating gate error is a chip level DRC and not enabled in the local DRC you're running. In any kit, when you run DRC on circuit level (not chip level), not all the DRC rules are enabled, for instance, the metal intensities, or the via arrays DFMs are not enabled, and the drc check does not report errors though they exist. So if this is the issue, then your drc without the full chip on was not clean, this PO.R.8 error was just not checked. Read your kit drc manual about the PO.R.8 to ensure you understand the issue.