I originally tried it in Firefox, so tried Chrome now and got it working. So tried Firefox again, and it worked. So it was just a skill issue on my end, and I had to be a lot more precise than anticipated. Sorry for the incorrect bug report...
But now that it did work for me, I think the major piece of feedback I have, is to give some kind of visual feedback to the player when drawing. Maybe also some animations on the character when you complete a level. But otherwise, I like the concept.