Currently all 3d widgets are composited onto an offscreen render target. So they’re resolved without any of the engines standard post processing, hence no glow. In the future we’re interested in supporting both, but for now, you will probably want to make sure the UI material is unlit, and you’ll need to get the glow by baking it into a texture you swap with the images you’re using on the widget.
This isn’t a very complex piece of UI though, using UMG for it might be overkill. You could achieve the same thing with a text component and a single quad with a material on it that has the energy bar and does all the masking and glowing. Would be much cheaper to render, and you’ll get all the post processing effects you want. Downside though from having it in the world is you’ll get all the motion blur, post processing as well. Though that’s true of the 3d widget component as well.