So what do you think is happening?
Are you saying the power displayed is wrong? If not for the same cadence I’m not sure how you think the resistance is increasing?
If I dual record with my pedals what would you expect it to show?
This is definitely what I also observe. ERG mode feels easier on climbs and harder downhill. I use a Wahoo Kickr Core smart trainer.
I kind of started doing my difficult workouts on climbs because of this effect. Yes, I am fooling myself and my coach does not know about it. I hope he is not reading here. 
My actual concern is whether ERG or Zwift power measurement is also wrong in general. The same watts feel much easier outdoors where I use a left-crankarm based Stages power meter. The discrepancy seems to be as high as 10%.
Just to give an example, Garmin estimates my FTP based on outdoor measurement as 244 Watts. But I can only hold it for 5 minutes at most with Zwift. On the other hand, my outdoor 5 minute power is around 287 Watts.
There are other issues, too. Try to hold a certain power level at a small gear in ERG mode. Then try it at a much bigger gear. ERG should be independent of gear selection but the feel is definitely not the same. I personally find the pedal feel much smoother at bigger gears.
This is due to the kinetic energy that the flywheel of your trainer has. With a bigger gear the kinetic energy is probably better matching the real outdoor case at that certain speed corresponding to the rider’s power output.