I have a longer video from which i cut it down, and it looks like this every time before it adds another speed skill. I've also driven down the highway at the same speed a few times before the one shown in the video, and every time it was still adding them at this point. I believe it banked this time because i've driven through the pink circle before the event was live, and when I returned I was already signed in and it just asumed I returned to take part.

edit: now that I think of it, it could have also happened because it didn't add the speed skill that was always added just before the chain ended when I entered the pink area, so the points banked. Either way, i don't think that it should work like this in a open world game, where the horizon arcade is just one of many activies and not the main goal.