It’s hard to tell without more information but, to me, it looks like you’re ’chasing watts’ (changing your cadence to try to hit the watt targets). If this is the case, just try to keep your watts constant and let Zwift adjust the trainer resistance so that you stay at the correct watts.
You need to pair the bike to Zwift for both resistance and power, it also looks like you don’t have cadence paired from the bike. If you still have your bike paired to the Roam then Zwift can’t control it and the workouts will not work in ERG mode. Get rid of the Roam and it should work.
ERG mode works during Zwift workouts, but the wattage doesn’t stay consistent like I see with other people using, for example, the Wahoo Kickr V2, etc.
I’ve seen others mention the same issue with the Wahoo Bike Shift on forums.
When doing workouts without ERG mode, I’m better able to hit the target wattage.
I assume you already have ERG mode power smoothing turned on in the Wahoo app?
Also, before you start your workout, shift to the easiest gear (i.e. 1-1). This was mentioned by GP Lama in his review video of the Kickr Bike Shift on YouTube as a way of improving ERG mode. I do it for every workout and it does make a difference.
However, due to the design of the Kickr Shift, ERG mode will never be as good as the Kickr Bike or Core etc. Wahoo have admitted this.
Shift works fine in ERG mode with Zwift, so its going to be something in your set up.
As others have said, only connect to zwift initially so you dont have cross talk and any other devices trying to control erg. Failing that, pedal smoothly and keep cadence fairly consistent.
1-1 is the equivalent of the smallest chainring and biggest cassette cog.
Obviously you don’t see the gear display in ERG mode, so when I start a workout and before I pedal, I just press the outer (the larger) shifter about 10 times so I know it’s in the easiest gear.
If I forget to do that then ERG mode is like what you experience.