That's odd. I think it was originally set to 25 fps (PAL) in the settings, although looks like I can force it to be 25.000. I have no idea how I can select "2 reference frames" though.

But are either of those likely to cause a video to fail to correctly process only the still photographs, but process everything else as intended? As I said, I've had previous vids with photographs in, which appear to be similar, but the photographs process as normal as far as I can see.