I don't think there's anything wrong with the car, but if you're curious then I'm curious too so I have 2 questions:
1) Were you in Econ, Sport or Normal (neither set) mode? As mentioned, in Econ mode it's easy to know if you accelerate enough to call for ICE assist, because you have to push past the detent. In the other modes you have to watch the power gauge to see if it's going past the blue range because that's before the detent. Personally I drive in Econ mode because it's easier to stay EV without looking, and the same power is available (flooring it fully gives max acceleration regardless of the mode).
2) When "EV" came back on, did the power gauge have the blue curved bar again? If it turned blue again immediately, that would point toward acceleration causing the ICE to come on (or regen-while-battery-full, but that's not likely by the time you got on the highway). If it stayed white, I think that points to a "system check". I've seen it stay that way for 15-20 minutes with EV on and off every few minutes before finally going back to blue bar and staying EV. My last one occurred after a run of 3 weeks + 1 day of no ICE use at all, but I think it's opportunistic and can happen a bit sooner. I suspect there are two thresholds -- if a system check hasn't happened in X days it will when the ICE comes on for other reasons, but if that doesn't happen then after Y days the computer forces the engine on for the check anyway.
The "HV" light only comes on when you manually select HV mode; it doesn't come on when the car naturally turns on the engine (EV range gone or acceleration).