Is there something WITHIN ZWIFT that can cause power to fluctuate? I can be riding on a Zero% grade, at a constant cadence, constant speed, HR, etc. but my power will bounce around and spike (e.g., running at 130 watts, jumping to 160, then coming back down). Have a replaced an older 2016 Kickr with a new model; getting the same issue. Not sure if it’s a setting, etc. will try anything at this point. Doesn’t really bother me much on solo rides or workouts, but it’s a killer on group rides, especially when going up a hill.
Do you have race mode enabled in the trainer settings? That will make power fluctuate a lot more.
Zwift also offers 3 second smoothing for displayed power (in settings) but that doesn’t change what the actual power is. It just makes it look more stable on the HUD.
Can you replicate the issue doing a ride in the Wahoo Fitness app?
Assuming you are not using Apple TV you could run a log through zwiftalizer.com to see if anything bad shows up there. Or analyze the FIT file to see if you can identify a pattern there and gauge how big the fluctuations really are.