I tried creating a new 4.7.3 third person shooter project and copying my Content folder into it and the same bug happens. This is what the Child Actor Class field shows when I first open the Blueprint:
When I open a different Map, the fields are set to “None”.
Another thing I noticed is that things such as deleting a folder (even an empty one) does not remain deleted when I re-open the project. Other changes such as adding or deleting a blueprint does work and get saved properly.