I have a clean room project where I placed an image inside a widget and then applied a Opacity Animation to that widget. I placed this inside of a Widget container and placed it in my scene. Rather than fading in as expected (and demonstrates in the widget animation preview), the widget pops after the opacity reaches 1.0.
This is working as intended, you will need to change your blend mode for the widget component. This can be found in the details panel for the widget component. Try changing this to transparent. I hope that this information helps.