I certainly won't be opening a bug ticket, as the problem was not due to a bug. I started with some reasonably complex code which produced results that I was not expecting. I then boiled that code down to a much smaller test script, and along the way I failed to correctly read and understand the associated documentation.

It was only after posting the question that I realised my mistake - at that point the reason for the seemingly inconsistent results became clear, and the results made perfect sense.