Just when I mark my PB for 15 seconds, I happily go to the event in Zwiftpower (ID: 1536921) to get a screenshot to brag a little, when I notice that my reported 15-sec power is very low. When I go to the power curve, I see that the first 8 seconds are completely flat:
The correct curve can be seen in my dual recording analysis, and it looks quite different:
The same is true for other riders on the same event:
I spotted the similar pattern in a different event (ID: 1542230), so this seems to be a widespread bug. Here’s a screenshot with the power curve for the top 10 riders of that event: