Vulnerability Groups : In Group configuration Filter Type field is not available

It is expected behavior due to "Handle filter type changes" and "Show or Hide Resolution tab" client script. These client script sets display to false if 'filter_type' is "Group rule" (value is "created_by_vgr"). 

Please see dictionary entry for filter_type:
https://<Instance name>.service-now.com/sys_dictionary.do?sysparm_referring_url=sn_vul_vulnerability.do%3Fsys_id%3D5d1c8425db967f40141ec9fb0b9619f9%4099%40sysparm_record_rows%3D153%4099%40sysparm_record_target%3Dsn_vul_vulnerability%4099%40sysparm_record_list%3DORDERBYnumber%4099%40sysparm_record_row%3D2&sysparm_view=&sysparm_query=name%3Dsn_vul_vulnerability%5Eelement%3Dfilter_type&sysparm_query_encoded=name%3Dsn_vul_vulnerability%5Eelement%3Dfilter_type

It is bringing "Filter type" if the value is other then Group rule:
https://<Instance name>.service-now.com/sn_vul_vulnerability.do?sys_id=bf7f0302dbe6f70041f0e39d0b9619db&sysparm_view=&sysparm_record_target=&sysparm_record_row=1&sysparm_record_list=filter_type%21%3Dcreated_by_vgr%5EORfilter_type%3DNULL%5EORDERBYnumber&sysparm_record_rows=6