I’ve noticed that Zwift does not do a consistently great job of controlling power in ERG mode, when I used PowerTap pedals. For my wheel-on Tacx Vortex trainer it does great. I think this is due to the software feedback controller used in Zwift, and shows up with the pedals because they report power ‘faster’ than the trainer. The fix would not be difficult, and I think a user-adjustable time constant that allows adjustment of the feedback integration time would fix this. See a post on Facebook with more details:
I should note that I’ve seen this same issue cropping up from other people for over a year, with the same symptoms. Zwift is also not alone. There are similar reports for Trainer Road (for controlling software), and also using Kicker trainers, and Garmin pedals, among other power measurement systems.