Custom Nav Area Not Affecting Nav Mesh Until Restart

When using a Nav Modifier volume with a custom Nav Area class, the Nav Mesh is not being affected until the editor is restarted.

Found in 4.11.2 binary. Reproduced in 4.12 Preview 1 binary and Main CL 2955029

1. Open the editor (TopDown Project)
2. Add a Nav Modifier Volume
3. In the Details panel, click the + next to the Area Class dropdown to create a custom area class
4. Compile the area class blueprint
5. Move the Nav Modifier so that it will affect the Nav Mesh (Notice that the color that you have set for the area class is not appearing)
6. Restart the editor
7. Move the Nav Modifier volume again

Result: After the editor restarts, the nav modifier volume is affecting the nav mesh as expected.

Expected: The nav modifier would affect the nav mesh immediately, without requiring a restart of the editor.