I am just starting with Zwift and noticed this problem immediately (still on my trial). In erg mode I can hold much higher watts than non-erg mode. I took two 5 min segments in the same ride. It was a 2:15 ride and the two segments were 15 min apart (last “over” segment and first 5 min right after the 10min cooldown ended) so added fatigue shouldn’t be too much of a factor.
avg in erg - 171w 133hr 76 cadence
avg non erg - 154w 149hr 76 cadence
The non erg power output was of course spikier than in erg mode, but the difference is massive for that small amount of power fluctuation. 17w higher at 16hr lower just seems like something is wrong.
I manually raised my ftp from what my test scored me and still felt the workout was too easy (SST Med). After the workout ended I felt like I didn’t get a good workout so kept riding a while. I guess I could just crank my ftp up another 20-30w after every test if I can’t figure this out, but would prefer it to just be accurate from the start.
Edit: Forgot to mention the trainer I’m having this issue on is a Kickr Snap.