I don’t generally use ERG mode as I find it unnatural and Wattbikes & Zwift workouts have never gone well together, but I decided to sign up to the Zwift Academy series and give the program a go. I’m riding a 2019 Wattbike and the first workout was a disaster. It started off with me in the spiral of death in the pens, which wasn’t a problem I had the last time I tried ERG workouts 3 years ago - I had to wrestle with the pedals for a painfully long time at very high watts and low cadences to get up to a working cadence & finally get the wattbike settled into the first target interval - I failed that star, obviously! On the 20s intervals the resistance changed too slowly to be of much use, and worst of all, in the “free ride sections” I was stuck in gear 6 (a very low gear on the wattbike) and zwift wouldn’t register any virtual gear changes whatsoever, so I couldn’t put any power down at all. Quit out after the first 30s all-out interval as it was basically a waste of time. Feel very disheartened that workout compatibility between Zwift and Wattbikes is still so poor years after getting one. It works so well for general riding, too 
I am having exactly the same issue - I reached out to zwift and they told me its wattbike’s fault for advertising a product as compatible with zwift when there are issues. Wattbike havent been much help yet either and keep asking me to recalibrate my wattbike (which doesnt help).
The wattbike is an expensive bit of kit and zwift subscription costs add up too - so it feels like both need to take responsibility here. Its incredibly dissapointing theres stil no solve after over 4 years.
All wattbike owners - please get onto zwift and wattbike to ensure they start taking this seriously!!
The gear 6 being stuck issue is a Zwift issue - they previously fixed it but broke it again.
The slow to respond to changes in erg is a wattbike issue - the older model has a stepper motor that simply put, takes it time to move the resistance to get the right wattage.
Though, look on the plus side, the V1 model is a hell of a lot more stable than the V2 model mainly due to the lack of ‘new features’ they applied to the V2.