Mobile viewports not producing expected results

I've been running into an issue where I can't produce references for smaller viewports. My specific situation is this:
- I have a casper script that gets run before the scenario. It waits for a selector to appear, then clicks on a couple of buttons before taking the screenshot.
- For a viewport of 1024 x 768, the expected screenshot is produced consistently.
- For a viewport ~<700px in width, a screenshot is produced but it is of a completely unrelated element on the page.

This is strange, because if it hadn't found my selector I would expect to see the placeholder image; instead, it claims to find it but the screenshot is just wrong. Changing the delay has no effect. Logging the output shows the element really is there in the DOM, so it's not a matter of being able to find the element under test.

Not sure if this is an issue with BackstopJS or the underlying ResembleJS, and I'm not really sure of the best way to debug this. I'm running on Windows and I'm also experiencing issue #242 so I can't debug via SlimerJS. Any ideas on how to figure out what may be happening?