Garmin measures 5k short, no PB for me!

I regularly run in the Parkrun here in the UK, which if you don't know is a 5k run every Saturday. I got my PB last week but my F5+ measured the race at 4.97km and so didn't log it as a PB - in fact most weeks it regularly measures the run 5-50m short. I edited the run and forced it to 5km but it still doesn't register the PB.

Any ideas how I can rectify this?

Thanks