Was there anything different between the ride where it seemed to work and the ride where it didn’t? Did you use the same device to run SYSTM? (and what device did you use?) Did you confirm that it was connected properly by checking the Devices list in Settings after you started the workout? Have you ever connected any other devices to the trainer that might have confused it? What you described (“all I could do to maintain the power was pedal very fast”) is exactly the behaviour you’d expect if you were in level mode. That makes me think that you might have been, even if by mistake.
It seems possible that the bike defaults to Level mode when it reconnects, either all the time or sometimes. If that’s what’s happening, you should be able to change it to what you want at the beginning of each video.
I’d do a test ride. First, I’d check to see what mode it starts in when you set up the connection. Then I’d test ERG mode to make sure it works (you can just ride for a few minutes and fast forward to some intervals–all you’re trying to do is make sure it all works). Then I’d pause and switch to Level mode and do the same thing again so you can feel the difference and make sure everything behaves as it’s supposed to.