I think you’re running into the same issue I had with this as described here:
Failing Full Gas training segments for using too much power - Bugs and Support - Zwift Forums
Basically, the game asks you to go full gas, but really it sets a specific watt target that is nowhere close to full gas, so if you listen to the workout and go full gas you will not get the stars. They could have modelled those full gas sections as ‘free ride’ sections to avoid this, but that’s not how it’s set up today.