If an *ebiten.Image that is not updated by Game.Update() is drawn with Game.Draw(), the image quality degrades (for text, external images (ebiten.FilterLinear), and vector graphics).
If *ebiten.Image is continuously updated with Game.Update(), the image quality does not degrade, but the load increases.
(There was no degradation when layering multiple drawings.)
By modifying the code to draw the *ebiten.Image that is updated by Game.Update() only once in the constructor or similar, instead of using Game.Draw(), the degradation is resolved.