No, the underlying flatgraph (which is used to generate the drawing commands) is cached and only updated on first call. Reset it by calling

  scene.updateFlatGraph();

after adding new shapes, before updating the graph & rendering

(see https://github.com/metafizzy/zdog/blob/master/js/anchor.js#L...)

edit: made an issue https://github.com/metafizzy/zdog/issues/21