I had a sudden nose dive on my Pint a month ago that really shook me. It was also on dry, flat, smooth pavement with no debris or sand. It happened without warning at 10MPH on my Pint, no pushback, no stability wobbles, nothing.
Sent the diagnostics to Future Motion to see if they could see any fault, turned up negative.
The only thing I can think of was I was preparing to cross a busy road and was looking ahead and behind me to look for a gap in traffic, so it's possible I was off balance and didn't know it, but I would have expected to feel some sort of wobble if I was.
I've never gotten back to the 12+ MPH range and i've shortened my rides as I still don't feel fully comfortable 100 miles later.