Two-Sided Material option is initially greyed out and inaccessible when creating a Material Instance

When attempting to change the option for Two-Sided Material for a Material Instance, the option is initially greyed out and you cannot change the value. However, if you close out of the Material Instance and then re-open it, the value will be changeable.

Tested in //UE4/Release-4.26 @ CL 15973114

1. Open any project
2. Right-click in the Content Drawer
3. Create a new Material
4. Right-click the Material in the Content Drawer
5. Create a Material Instance
6. Open the Material Instance
7. Enable the Two-Sided Material option

RESULT:

The Two-Sided option is greyed out and the value cannot be changed

EXPECTED:

The Two-Sided Material option isn't greyed out and the value can be changed