Retainer panel causes textures in 3D Widgets to appear washed out

When adding a widget with retainer panels into the world via a WidgetComponent, the texture appears washed out. This does not occur if the widget is added to the viewport, nor does it occur in 3D widgets without retainer panels. The washing out only seems to happen at runtime, and occurs even when all lighting is removed from the scene.

1. Create a widget blueprint with an image wrapped in a retainer panel
2. Apply a texture to the image
3. Back in the level editor, add a widget component and apply your widget
4. Start PIE
5. Texture will appear washed out

[4.18p2] Retainer panel causes textures in 3D Widgets to appear washed out

UMG retainer box washing colours out on 3D Widget