We are both making a fair amount of assumptions about how these devices operate. Riding flat and level constant cadence there are times where I see all sorts of fluctuations in the GPS displayed speed. I also see fluctuations in how long it takes to update the speed, there are times it is noticeably longer than a second. the idea these units are updating everything at a fixed interval is a big one.

Reading through the OPs situation again where everything looks good up till the end of the circle, my guess is the unit is trying to use some sort of long term averaging to compute speed and doubling back on oneself is confusing it.

At any rate have to agree with the second post, add a wheel sensor.