Unable to Edit Default Struct Key of TMap

In a TMap where the key is a struct, it is not possible to edit the default value of the struct.

Found in 4.15.0 CL# 3299760. Reproduced in 4.16 Main CL# 3345747

1. Open the editor
2. Create a new struct (TestStruct)
3. Create a new actor blueprint
4. Add a TestStruct variable to the blueprint and change it to a Map variable
5. Compile the blueprint
6. Add a Map Element in the variable's Details panel

Result: Notice that you cannot edit the struct's default value.

Expected: It would be possible to edit the default value of the struct as you can with another type such as an integer or float.