I ran into this a few years ago, and here it is, back in 2019.

The assembly that shows at the top of the dialog is the assembly that these operations were broken from. There should no longer be any reference back to that assembly. However, if I try to edit this component, where I would be adding an external reference to the NEW assembly, it circles back to the original assembly, which no longer exists, and that it should no longer refer to, because all the references are broken.

When I try to add a new reference, the message I get says the part is locked, because of the reference to the old assembly. As you can see, there is no opportunity to unlock anything here.