I found a bug in ERG mode. I have been running more lately and after running, the next time I ride on Zwift ERG mode is switched off. Additionally when I switch it back on in the Training section and press the back button (for example when I want to do a group event workout and not actually select a workout for a solo ride) ERG mode is not actually enabled. It is only after I select a workout and then leave that solo workout can I join a group event workout that ERG mode stays on. Hope you can reproduce it.
Edit: My last run included a Zwift workout as well.