Increase/Decrease Tension

I am new to Zwift - I am using a Tacx Vortex Trainer - I have noticed when I simply select Ride - my workout tension increases/decreases with the gradient of the course (which is awesome) - however if I select a workout (i.e. I did the McCarthy workout yesterday) my tension stays the same all the time - however my speed will drop and increase based on the gradient even though I am pedalling at the exact same cadence - I have tried to reach as much as I can on Zwift posts so I have learned a little bit about “ERG” mode - I checked yesterday and “ERG” mode is not on.  I am not sure if there is something else I may be doing wrong ???