There’s more info here : "Free ride" sections of workouts don't disengage ERG mode on Wattbike Atom - #53 by Craig_Stow_Charlotte
It’s a wattbike / Apple TV issue that has been around since launch.
If you have a IPad or pc/ant+ Option this will work around it…
There is a further issue with workouts being erratic on Apple TV while using erg, having a pc/ant+ option as back up
Is useful…