Both yesterday and today I had this problem.
Part way through riding in workout mode, my heart rate dropped down to 0 and stayed there. Not too long afterwards my trainer wouldn’t change resistance based on the changing workout. I got stuck at 200w and no matter how slow my rpm the watts wouldn’t drop down to the required rest zone.
I haven’t had any issues prior to yesterday so I can only assume this is related to the most recent Zwift update.
You can clearly see where the HR drops out completely in the screen shots from both rides. It also becomes impossible to reduce/increase power as it seems that ERG becomes fixed at some point.
FYI: Running on a Wahoo Kickr Snap