I deliberately looked for this on my ride this morning and mine's doing it to. Interestingly, it'd lose 1 km/hr in the last metre or two before I stopped, then 1 or 2 more as I moved off but was back up to its previous figure by 20m from the stop. You have to really look for it to see it because when stopping and starting you're usually occupied by riding the bike.
I keep a spreadsheet of my rides where I enter the distance and time and it calculates the average and that calculated figure always agrees with the Bolt's average.
I think we can just put this down as an interesting quirk.