SceneCapture2D Render Targets do not retain Alpha when in Rendering Preview for Vulkan and ES3.1

Regression: This issue does not occur in Release-5.0, so this is a regression

The widget completely obscures the screen only when switching Preview Rendering modes without looking at the create Blueprint first. This is potentially not part of the issue, but expected behavior as the SceneCaptureComponent2D needs to recapture the texture in the new rendering mode.

1. Create a new FPS Blueprint Project
2. Create a new Character Blueprint Actor 
3. Add a Camera and SceneCaptureComponent2D components to the Blueprint
4. Set the Mesh (tested using TutorialTPP)
5. Make sure the SceneCaptureComponent2D camera is facing the Mesh
6. Set the Texture Target in the SceneCaptureComponent2D to a new Render Target
7. Right click the Render Target and create a material from it
8. Set the Material Domain to User Interface and the Blend Mode to Translucent
9. Set the RGB of the Texture Sample to Final Color
10. Add a OneMinus node
11. Connect the Texture Sample's Alpha to the OneMinus node
12. Connect the OneMinus node to the Opacity
13. Create a new Widget Blueprint
14. Add an Image and set the Brush Image to the new Material
15. In the BP_FirstPersonCharacter, create a Create Widget node and connect it to BeginPlay
16. Set the Class of the Create Widget to your created Widget
17. Create an Add to Viewport node and connect it to the Create Widget node
18. Change the Preview Rendering to Android ES 3_1 or Android Vulkan
19. Play the level

Actual Result: The Widget is completely invisible (or if you haven't reopened the new Character Blueprint it will completely obscure the camera)

Expected Result: A cutout of the TutorialTPP will appear on screen and the space around it will be empty