As your chip didn't register a time for any of the 5K splits, it probably completely failed sometime between the check at the expo and Sunday morning. That can happen if the chip is passed through a strong magnetic field or exposed to strong microwaves. Presumably nothing you did intentionally, but there's lots of magnets around whilst riding on escalators, or placing the chip next to a TV/speakers.
Maybe there needs to be a way to confirm the chip still works at Greenwich on Sunday morning, although the queues for the toilets are long enough without another queue for chip checks. It probably makes sense to keep the tag away from possible magnetic sources until Sunday morning, e.g. don't wear it on your shoes whilst traveling on the underground.
As you're clearly on the photos, hopefully VLM will make a special case for a GFA entry for next year.