I meet the same problem. I magrate my old project to new engine version UE 5.1, and still use C++ spawn blueprint actor. It works all in “Standalone Game” Mode, but after packaged, c++ can’t spawn blueprint by path.
I guess the may be the new engine version UE5.1 only package assets in world, but that blueprint actor only in content folder.
So I change the default project settings. Project Settings>Project>Packaging>Advanced>Cook everything in the project content directory.
After it works well in both editor and package.