iPhone 15 Pro, and PiP

I’ve just got an iPhone 15 Pro, and PiP was working fine for me yesterday but today the option just isn’t there. It was in the top left by the X but now I just have the zoom option

Is this a common glitch, or related to file type or something?

IMG_5058

EDIT: My mistake, I didn’t realise PIP was impacted by the codec. Appears that VC1 mkvs aren’t able to be played PiP, but x264 are