There is a custom property set for each configuration, but also a master set for the file itself. in PDM this is on the @ (part symbol) tab.
This is the custom property data set on the custom tab, rather than the configuration specific tab.
This is different from the default config custom properties.
It seems to me so far that custom properties tab builder has no way to access this property set, only the configuration specific data sets.
I really really really hope I am wrong and missing something, because this makes the custom property tab builder rather useless to me because of how the data is applied in may legacy files.