Zwift ride suddenly feeling heavy in the middle of the ride

My wife and I both have Zwift Rides with Wahoo Kickrcore2. We ride side by side. During some of our rides, at first we were in the same pace, And then I notice that she gets left behind and then her cadence slows down and telling me that her Zwift ride feels heavier even if she shifts her gear down. When I tried her Zwift ride. It is in fact a lot heavier to pedal compared to my Zwift ride. So I knew there is something wrong with her bike. Good thing there’s a way for us to compare the 2 bikes.

Anyone experiencing this?

Is this in Erg mode?