Why do some window sizes report 'out of bounds' when running the update command?

I have compositeImage: true in my conf file, so I would expect that it would work in any window size.

For example, I am trying to capture my footer element.

Gemini works at these window sizes:
320x640
600x800
960x1024

But fails 'out of bounds' at:
480x800
768x1024

Is that a bug, or am I misunderstanding how it should work?