Yes it's because of this change: https://github.com/laurent22/joplin/commit/9aed3e04f43e9e02cd727d6f05a785e4aedad412 which allows updating the resource metadata and content independently. It will be needed for the OCR feature, so that users don't have to re-upload the content of all their resources after we attach the OCR info to the metadata.
So now if we want to modify the content of an existing resource we need to call this "updateResourceContent" method, which updates the right timestamp, and unfortunately I forgot that the image editor on mobile needed to do that.
I'm going to make another beta release of the iOS and Android apps since this can be a major issue for those who use js-draw.