I got this to work for my last workout, and here’s what I did. Opened zwift and paired everything, making sure “Controllable Trainer” was unclicked. Opened trainer road and made sure everything was paired, and started workout. At this point trainer road erg mode was not in control as I started riding. I closed zwift and continued riding on trainer road, and within 15 seconds or so, trainer road erg mode took control and kicked in. I then re opened zwift and quickly clicked past the pairing screen, as to not allow it to auto select “controllable trainer”. Everything worked from here on out in the workout.
For reference, I am using ANT+ stick on laptop running trainer road pairing to hammer trainer, and Apple TV running bluetooth for zwift, pairing to hammer trainer.