Distance wildly off when tracking indoor walking

Tracking for indoor walking has always been okay for me. Since yesterday however I am getting ridiculous distances when tracking my indoor walking. Yesterday my Vivoactive 4 recorded 6.68km in 33 minutes, and today it recorded 4.6km in a similar time. So not only is it inaccurate, but not consistently so. My walking pace on the treadmill was set to 4kmph. Have tried turning it off. Has anyone else had this issue? I've not changed any settings and as far as I know there's been no software update.