This is not the first time ERG mode has stopped working during my FTP Builder Training, so while fresh off the bike I will tell you about the ride today. I was in Week 6 day 2 and when I got to the section where I have 150 W for 5 minuets then 2 min @ 80 W, every time it went to the 150W segment I could feel the resistance build. I was doing 150W with 90-100 RPM but I had to be in the easiest gears to do that. My heart rate was also another factor that told me I was working much harder than usual for that wattage. Like I said this has happened multiple time during my other training session of the Beginner training programs. This gets very frustrating.
If you can provide a bit more information, we may be able to get to the bottom of this:
What trainer are you using and is it properly calibrated?
What other equipment are you pairing with Zwift?
Regarding ERG mode: it is meant to hold you to that wattage no matter your cadence. If you are using high-cadence, then it will release resistance to compensate. The opposite with low cadence, it will increase to make sure you hold a steady 150 watts.
I’m sure you know this already but it sounds like it’s working as intended. If ERG mode stops working then all resistance is lost OR it goes into SIM mode, which simulates the grade changes of the course itself.