Oh wow. Could it not be the improved riding position / bike setup of the new bike though rather than the electric assistance??
I get knee pain too if really hammering the bike in awkward positions which is all to do with bike setup/positioning...
Not saying that it's not the electric assistance just playing devils advocate from my own experiences.
I think it's a bit of everything.
So, the knee pain wasn't caused by cycling.
When I was on my racing bike (after a year of not using it) it actually hurt my knees. I assumed it was the force I applied to the pedals going up steep hills.
I assumed the ebike helped as it 'flattens the hills'.
But yes. It could well be position too. The mtb has a bigger frame..
After 2 rides that constant annoying pain was just gone.
Had x-rays and apparently no loss of cartilage. So no official diagnosis. But remedy is always 'physio' which has been very hit and miss.
Genuinely thought that was my life, constant knee pain and eventually replacement, but this has at least put that off for (hopefully) a long time
So this bike is as close to a miracle as you can get. For me anyway.
Below is a pic of my miracle bike