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)
We have now released VideoScribe version 3.2.0 where any transparent strokes will be ignored during the morph so you will no longer see the black lines over images set to morph that have a transparent reveal stroke over them.