Step tracking is accurate, distance isn't

I am having having a weird issue with my Epix Gen 2.

Step tracking is very close to what two mobile phones and a Fitbit are recording but distance is wildly inaccurate.

Correct distance: 15.2km; Garmin distance: 20.3km

I have noticed this behaviour over the past one week, when the Epix 2 has been consistently reportage steps correctly but not the distance covered.

Any tips?