For some time now, I have not been able to select my derived configurations in the assemblies. Only the configurations of the 1st level appear.
As can be seen in the attached screenshot, I inserted my 12552-COR-01 set into my assembly with the "Function" derived configuration. However, if I want to change the configuration, I have access, through the drop-down menu, to the 2 configs of the 1st level (Gold & Steel), but not to the ones that are derived.
To get everything back to normal, simply change the order of the configurations in the list with a "Click and drag". Apparently, moving a configuration is enough to restore proper operation.