Hard to tell without seeing the whole code, but my theory would be this:
(a) There's a 4:3 /canvasaspectratio defined in the script's <defaults>.
(b) For some reason, said canvas region is *left-aligned*, either explicitly via /canvasposition or due to a glitch. The grey 4:3 overlay illustrates the effect.
Try explicitly setting
/ canvasposition = (50%,50%)
and let me know if that resolves the issue or not. Thanks!