Kind of an E-bike newbie here myself too...but I noticed the same thing on multiple bikes that I tried. I did end up buying one and was feeling it too, but I think I've wrapped my head around it.
The motor is either cadence-sensing or torque-sensing (or both). If you stop, even for a second, there is neither cadence or torque to sense so the motor cuts off. When it stops, it feels almost like it 'jerks' or 'pulls' (when I first felt it, I thought the brake was being mildly applied). But all that's happening is that the motor stops and suddenly without the assistance, the heaviness of the bike with no motor (especially if you are going uphill) suddenly feels like it's pulling BACK.
I think that's what is going on when I felt it...so try to determine if that may be what is happening with you too. Pay VERY CLOSE attention to when it happens. Did you "just" stop pedaling that moment?