I think it's just the nature of the bike. The rear is really heavy compared to the front and the motor has a lot of torque. Maybe try weighing the front down some, although that can also create new problems with the front wheel wanting to whip around when you turn.
For what it's worth I find it hard to not spin the back tire on loose stuff sometimes, at least occasionally, and the front can really "float" some on a steep uphill that is looser dirt. I just lower the assist and plan/compensate for it if I can't avoid those areas. I usually just avoid them.