So you are now using 1.8.23. Could you please use 1.8.36 instead? There have been bug fixes since 1.8.23.
What is your call to Diagram.makeImageData?
Do you have any clue why the Canvas sizes are different on their machines? Could you debug on their machines to see whether there is any CSS that is affecting the Canvas element or any Div element within the Diagram’s Div element?