I have a field for image upload. The upload works fine, images are uploaded to "/images/mysubfolder/123/myimage.jpg" - where "123" is the article id. For storage I use JSON / Article / images[image_intro] to get it stored just like a Joomla! article intro image.
Now there's something strange: Sometimes everything works perfect, but sometimes the path stored in the database loses the article id. So the path is stored as "/images/mysubfolder/myimage.jpg", but the image actually is in a folder with the article id.