Our engineers have identified it as an issue with the Photos app after we changed the background to be transparent instead of opaque in the previous version (3.2) of Draw. Although it doesn't sound like we'll move back to our previous opaque background, I do know that this discussion will continue with the Draw team.
A couple of things:
Instead of Save Image from the Share menu, choose Copy to Clipboard (from there, try pasting it in Notes or email or some other app). That should save your art with a transparent background.
Also, there is a workaround for this (putting a white background in a layer) to avoid the transparent background issue:
1. Create a new layer in Draw and move it to the one right after the Photo Layer.
2. Draw a square closed object that's large enough to be the background for the art (I zoom out of the canvas until I can se the whole thing and create a square just inside it)
3. Fill this closed object with a white background.
Hopefully that helps.