Just posted on exactly this topic.
https://www.radowners.com/index.php?topic=790.msg3919#msg3919

I think it´s normal. My 2018 Radwagon has done this since new and I think it´s the way the controller is programmed. Just what it´s doing I´m not sure as when you get this drag going downhill it does not seem to be regenerating energy into the battery.

My 2018 European Radwagon does this thing where you´re coasting down a hill and you´ll feel a slight drag in the motor that comes and goes. It only happens if your speed is approaching or exceeding the maximum set speed of the controller. To be clear, this is without touching the brakes and activating the regenerative braking; it seems to be braking mildly all by itself, but the regen display shows nothing. It´s puzzling, and also annoying at those times you want to make the most of a long descent at speed and the bike won´t let you.