There's an option in the converter for a while for Metalness where you can force it to never set a material to Metal, for those situations where many materials are converting incorrectly. Unfortunately due to the original materials having physically incorrect settings, the converter may never be able to know for sure what was meant to be metal and what was not, and whether or not things can be improved there is this setting in the converter in the meantime to ensure scenes where many materials are coming in wrong can avoid the problem (leaving you to only set those things that are metal to metal afterward, which should be less work at least!)