I do not get thumbnails for MP4 files if the Rate Control for x264 config is set to 0 (High Quality). Is this to be expected?
Definitely not expected, but I can confirm that at least on my MBP, the hardware decoder in Intel Iris Graphics 540 cannot decode H.264 encoded by x264 in CRF mode with quality set to zero (it succeeds with quality set to 1 or higher).
I tried this on macOS, but I suspect that this is purely a hardware limitation while the desktop environment doesn't offer a software decoder as fallback. Presumably a problem between libx264 and the hardware – I don't think there is anything we can do here.