It’s the tip of the front wheel for rider B.
Once again, from the tests I’ve been running, simply tweaking the system so that riders are exposed to no draft earlier is not enough for a decent speed reduction. I know it totally sounds (and is) more realistic, but due to the momentum carried from behind the deceleration caused by the air drag is too slow to make a noticeable effect.
on another topic, some days ago someone came up with the problem of coasting and being spit out the back in groups like RoboPacers, etc.
I did found the issue, there is indeed some braking going on when your power drops below 10w, coming from some old code 
The good news: it won’t be a problem after the next release.