I have had this exact same thing happen to me. Gently accelerating at 7.5 mph, board fully disengages with no warning whatsoever. My feet were extremely solidly planted, there were no sensor warnings in the app (and it did notify me of the crash), and there was nothing but smooth pavement in my way. As someone who has made it to 26.4mph on a original OW I can guarantee this was not user error.
I'm now going on around a month of combined time that FM has had my + in for repairs, and as far as I know they still don't have any idea why this is happening. Two separate trips back to them. This time they have had my + for 2.5 weeks and aren't even responding to my emails about it. I used to be a huge fan of FM and OW, but this has completely soured my experience. I just want my money back.