I have the same issue as you with 2020 Bosch gen4, Kiox, 625w battery. Can we discuss separately, not taking space in this tread?@Steve b , you’ve described, almost, what happened to me. (Bosch gen4 2020 Rail 9.7, Kiox, 625w, similar mileage).
Motor cut out 20min into my ride, usually in the same spot (gravel access road). I could count on this happening, at this stretch of gravel road, almost every time. Not too steep, not rough, but not smooth either. The cut off did happen in another area, once, while I was passing another eMTB. Never on a decent or a rock garden climb.
When I would lose power assist, I usually peddled through it until power returned. If the Kiox completely lost power then I would have to stop my bike and restart the Kiox. To me this was a mystery. I had the latest update, motor was quiet, no weird rattle, wet-ish environment, rode almost daily for 15-25kms, full charge each time, (fresh underwear each time). I was searching for anything. Everything looked fine.
All the suggestions here are possibilities and this forum is where I finally found a solution. Poor battery connection. The fix suggested was to slightly reposition the battery connection plate (1mm maybe) by loosening the plate bolts and prying it slightly closer towards the battery. Retightened and checked for battery fit.
Problem solved. Never had a vibration frequency induced cutoff again. I’m not sure if I got lucky but I could have searched other possibilities first and eventually made it to the connector issue. (I also stopped searching for a buried Flux-Capacitor in that spot ).