I have this issue with multiple, unrelated assemblies. Sometimes when I am modelling a part (Part A) in context within an assembly, I will need to make a slightly different version. To do this, I typically have the assembly open, then I open A and save it as a copy (B.) I then close Part A. Going back into the assembly, I click Insert>Component>Existing Part /Assembly. With B already open, it auto populates B into the feature manager.