animate >AME exported video with some scenes in opaque color

Animatecc exported via AME my animation video in good color except for several contiguous images/shots with color that was opaque from the rest of the H264 video.  My Animate timeline images have  consistent color values; no opaque images.  I troubleshooted and discovered after several hours that I had tried to put 'video slugs' before those same shots by keyframing about twenty frames before each of my images/shots.  In those keyframes I selected the rectangle tool and a black color and created a black canvas for the sets of frames.  Thinking there was a connection with my problem, I went back to my animation in the timeline and removed them.  I exported my animation again via AME and the result was an H264 video with consistent color throughout that matched my timeline animation.  What's going on?