Structures don't save map elements that point to an asset reference

Adding map elements to a structure does not retain the information after saving and closing the project. This only appears to happen after you add a considerable number of Map containers inside a structure like 4 or more. With few it works fine.

1. Create some actors
2. Create a Struct
3. In the structure add some map members referring to the actors you created 
4. Save-All
5. Close and reopen the project 

Result: Opening the structure again will show the member that uses Map container with broken values like "PLACEHOLDER_CLASS..."

Expected: The Struct to save properly after saving and closing the project.