Canvas Render Target 2D doesn't get alpha mask when rendering with ES3.1

The Alpha Mask on a Canvas Render Target 2D no longer appears when rendering with ES3.1. This is a regression from 4.24.3 (CL-11590370).

This was reported and tested in 4.25 Preview 7 (CL-12759524) and reproduced in Main 4.26 (CL-12945139)

1. Open the UE4 Editor and create a Top Down project
2. Create a CanvasRenderTarget2D asset in Blueprints folder. Use the default settings
3. Create a Blueprint Actor and open it
4. Drag in the TopDownCharacter to the SceneRoot and add a SceneCaptureComponent2D Component and position so you see the character
5. Use default settings and set Texture Target to your CanvasRenderTarget2D asset
6. Compile and save
7. Make sure you see Character in the Blueprint Actor Viewport, then double click on CanvasRenderTarget2D asset to confirm the mask
8. Change the Preview Rendering Level to Andorid ES 3.1 and repeat step 7.

Results: There is nothing on the CanvasRenderTarget2D. Alpha Mask does not exist

Expected: To get Alpha Mask in Canvas Render Target 2D