The NGC netlist which is a submodule within another NGC is treated as an unreferenced source and is not included in the design hierarchy.
The following critical warning appears when opening the synthesized design:
Critical Warning:[Project 1-485] Could not resolve non-primitive black box cell 'yyyyyy' instantiated as 'xxxxxx'.
At the same time, an "Undefined Modules Found" dialog box pops up with the following message:
While importing this netlist, 1 undefined cell was found and converted to a black box.
Make sure you have loaded all intended module definitions before proceeding.
Black boxes can be populated by using the Add Sources operation and re-running Synthesis.
Module name: yyyyy
This is expected behavior as the Vivado tool does not read the content that the NGC netlist sources when creating the design hierarchy, so it does not try to find the nested netlists.
To resolve this issue, you can use one of the following solutions: