Why does Sketch sometimes resize non-proportionally while holding shift?

In my Sketch document, I have a picture that has a similar aspect ratio to some other things on that page. If I select both the picture and a shape and then resize while holding down shift, the resize is not always proportional. At some sizes, the red guides will appear and snap the image size to match something else on the page — this breaks the proportions I'm trying to keep.

Here is an animated gif showing the behaviour. Note that the kitten is often distorted at smaller sizes. (After I made this gif, I thought the problem might be that the hand is a symbol. I tried doing Detach from Symbol, but the resizing problem remains.)

What's going on here, and how do I keep the image proportional at all sizes?