Saving a multi-frame GIF gives nonsense

[Nyle]

Saving a normal image as a GIF works fine, but when I try to save a three dimensional array as an animated GIF I get an animated GIF that seems to have little relation to the image I'm trying to save, and even changes each time I save it. A minimal example of the issue:

using FileIO, TestImages
img = testimage("mandrill");
save("mandrill.gif", cat(3, img, img))

This will give a different image each time the save line is called with, at best, only vague resemblance to the original image.

If I instead save as a non-animated image, everything works as expected

using FileIO, TestImages
img = testimage("mandrill");
save("mandrill.gif", img)

I'm on a Mac, 10.11.6, using Julia Version 0.6.2. I'm not sure where to find version numbers for packages, but I called Pkg.update() earlier today, so everything should be at the latest version.

Note: I'm fairly certain that FileIO is calling QuartzImageIO to write the file out based on its registry file, but am not sure how to verify this.

Edit: It seems that while my computer displays the images as two rapidly alternating frames, my browser only shows the first frame; I suppose this might be indicative of some issue with how information about the two frames is encoded?