Initial height & width always the same

In the latest .dev version, when trying to set the initial height and width of the shadowbox, the values can be independently set and saved, but when you go back into the settings, they're both set to the same value...

Example: I want to change the initial height and width to 480x640 (respectively). I go into the Animation settings, and set the two values; initial height: 480, initial width: 640; save. I then test opening a shadowbox-enabled image, however the initial box that opens is a square - height and width are the same. I go back into the Animation settings and check the values - sure enough, they've both been set to 640.

I looked through the code to see where this was happening, but couldn't find anything... Anyone else having this issue or know what could be causing it?