If you were at 50-70 km/hr, you did not have a 'full' battery (You used some of the battery to get up to those speeds). This problem is most prevalent with people that leave their garage, and go down a big hill: The battery is absolutely full (100.0%). If I can get the battery down to 96%, I do not have the problem. I have also had the engine start in panic stop situations, with the battery at lower charge.
50-70 km/h is 31-43 mph. Doesn't seem like it should take that much power to get to those speeds. I think people may have reported the regen ICE start at similar speeds.
I've never actually experienced a regen startup; in fact I don't believe I've ever had the engine come on when I didn't know exactly why -- acceleration past the detent or in non-econ mode, EV range at 0, or I had put it in HV mode. When it comes on, it seems to stay on until the coolant temperature hits 160, which varies based on outside temperature but averages 5 minutes. RPM was right around 1500 the whole time during this episode, which seems like idling. That also bolsters the theory that it wasn't extra acceleration that triggered it, and again she wasn't braking yet at the time so probably wasn't related to that. I don't really have anywhere I can charge then go down a big hill, but will try to get a regen startup if I can.