In fact, I think the problem isn't a bug of your app.
I used it under CHROME and FireFox to generate and export an image file.
With the json file (and no change between the two tests) :
-> I don’t have the problem under FF
-> but I reproduce it under CHROME.
Attached : index.png (FF version) and index2.png (CHROME version) to see the difference…
If you want to investigate, I can share my json file with you. Let me know.