I found it to be a coding issue with the Priorities.XML file against the custom_RunFaster_Metavariant_metatypes.xml file, where the Nartaki are listed as (Human) in the Priorities file, but are (Human, 4 Arms) and (Human, 6 Arms) in the custom metatypes file. Chummer5 is seeing this as three different metatypes, allowing one that doesn't exist and disallowing two that do because they're not listed with a Priority value.