During the animation part of a morph, only stroked paths are visible and they are always black and 100% opaque. That is just the way morphs currently work and it is not a bug.
You are correct that you are seeing your transparent strokes turned black and 100% opaque.
Morphing works best with SVGs that contain nothing but black stroked paths (no color fills and no embedded images).
For that particular lawnmower image, a better method might be to use "move-in" (instead of drawing it) with "no hand" to move the image in from the right hand side of the screen to it's final resting position on the left side.
-Mike (videoscribe user)