Confusing being high for low?

I went on a run today with my diabetic brother (13). Not long in, he started feeling low. Dexcom G5 showed 12.1, which continued to go up to 13.5 15 minutes later. We thought this was a sensor issue, but when we got back home (him still feeling a little low) and took a manual test which showed 17.

Anyone else had an experience like this?