THANK YOU very much for very quick abd quite correct reply!!
I just jumped on here with the same qquestion/problem! "Expand the Mesh"... perfect.
I have a nested comp (a trombone players head, arm, hands and instrument) that are looping. I wanted to apply the puppet tool to his body to have that move a little as well, but not as complicated as the "head" comp. However, every time the nested comp moved outside the bounds of frame 1 (where the puppet tool had been applied) there were little artifact outlines of the initial frames shape. Hard to explain, but expanding the mesh to include the whole range of motion removed the artifacts.