I suspect that based on the lack of response, this is something other forum members haven’t experienced with their mk8 Golfs.
I think that VW’s default position with the driving data resetting is that the long term data resets to zero after every 100 hours of driving. Could it be the 100 hours of driving time that has triggered the data being reset at that mileage? 100 hours of driving in 3,866 miles gives a long term average speed over that distance of 38.7 mph, so onice you‘ve travelled a few hundred miles since the data last reset itself, it might be worth checking to see what your average speed is - if it’s somewhere in the region of 35-40 mph, then i suspect you‘re travelling 3,500 - 4,000 every 100 hours of driving time and your car’s computer is merely doing the ‘normal’ driving data reset.
Edit; just checked the long term driving data in my car (2020 Polo GTI+). It’s currently showing 97 hours of driving so it’ll reset after another three hours driving. My long term average speed for those 97 hrs of driving is 33 mph, so based on that figure 33 x 100hrs means it’ll reset back to zero at around 3,300 miles since it last reset. My driving is approx 2/3rds ‘A’ road driving; usually at 50-60 mph and 1/3rd town driving, so for you, an average speed of 38/39 mph sounds reasonable for 100 hours / 3,866 miles of driving if your mix of driving is broadly similar to mine.
Edit; my car’s ‘long term’ driving data reset itself yesterday as expected as I hit the 100 hours driving time. The ‘since refuelling‘ and ‘since start’ data does not rest at the same time; these will reset as explained by
@T-bro in their post below.