How to unload/unlock a implicit-linked delay-loaded dll?

Sorry for my bad English.

During PIE, I can successfully unload my implicit-linked dll by calling __FUnloadDelayLoadedDLL2, and consequently the dll file is no longer locked by the operating system.

However in my editor module, calling to __FUnloadDelayLoadedDLL2 does not unlock the dll file.

The possible reasons I considered are:

  1. The UE4 editor does something with my dll, so it prevent me from unlock the dll file.
  2. The calling thread of __FUnloadDelayLoadedDLL2 is wrong.

What is the actual reason I can not unlock the dll file used by editor module?
How can I successfully unlock the dll from editor module?