How can use Materials from a Pak file in editor?

Hi,everyone! I made a PAK file with some Blueprints and Materials(sth like M_Outline) in it from Project-A(Blueprint Project).
Then, In Project-B(C++ Project), I wrote a Plugin to mount the PAK file and load assets(Blueprints & Materials) within it successfully.
I can directly use blueprints from the mounted PAK file just as the same as local BPs in the project,
But I can’t use those Materials from PAK file in editor,
in other words, the editor didn’t recognize those Materials in PAK file even though the plugin already loaded them with method LoadObject().

What need to do besides “LoadObject()”?

Any one can help? I use UnrealEngine 5.0.3, BTW.