So.... why does THIS distortion happen?
I've had two or three clips (out of hundreds I've done) exhibit this annoying "breakup" of a part of the composited image. (see the stormtrooper 3/4 -7/8 of the way thru)
The "commonality" in all the clips that exhibit this defect is that the part of the image that distorts / breaks up is green-screened on "top" of the supporting footage. Sometimes the support footage "behind" the green-screened video clips is stills (jpg or png) or sometimes it is also video.
This distortion does NOT show on the "preview" in the lower right as the clip is rendering, and it does not show up in a full quality playback or RAM render on the editing timeline. It only is there when I play the composited clip.