I first noticed this odd behaviour when trying to animate a switch between its on & off state. Smart animate partially dissolves (there’s some faint ghosting) as well as pixelates the stroke for the duration of the animation. Here’s some more testing I did to try and replicate the problem.
This is a unique switch where the knob transitions between a rectangle and a circle. I’ve tried narrowing in on the exact set of parameters that leads to this buggy animation and it seems to consistently appear when the corner radius of a rectangle changes with stroke applied. You can see in my examples that this doesn’t happen with only fill applied on the rectangle, but does appear when they’re outlined with a stroke.