I was not focused so much on the start of the race which is different than in race. What I said before still applies during a race, group ride, etc… The start of the race is a bit different as some of the others have pointed out.
I see the same thing as others when viewing the data via Strava. First minute of Tour de Zwift - Stage 9 (Long):
I typically get my power up a bit with 5secs to go. After that, I do what I need to make the front group. Depending on the race, that could be speed up, slow down, or stay steady. Anyway, I always have cadence, HR, and power from the very beginning. The speed appears to be zero until about 6s after which it shoots up, but if I zoom in on the first 3s there is still an avg value but w/out a peak value. One would have to look at the original .fit file from Zwift to determine if it’s a Zwift problem or Strava problem. Either way, I would say it is a cosmetic issue and unrelated to which trainer you are using.