Fan speed issue. ERG Mode - fan stops below 120W

As above I’ve set the fan speed to HR mode in the app, after previously trying it in Power mode, but every time I start Zwifting the fan speed goes back into Power mode. As I mostly use the bike for Workouts it means if Im doing  a big interval with a 50% of ftp recovery phase the fans stop immediately as my power output drops.

Yet once I quit the workout and exit Zwift the fan comes back full blast until I disconnect my HRM, which is what you would expect of HR mode.

Any ideas?

** An update on this ** I realised after posting this that its when its in ERG mode and the power target drops below 120w it does it.   It doesn't do it in Incline mode.  If you have Zwift you can see it happening if you switch between ERG and Incline mode using the keyboard or the Companion app.