CHG MODE Looks officially... GONE =-/

If the battery is not depleted, HV mode will keep the charge level around around where the car was at when the mode was selected. On that first hit of the gas, the charge was likely at the low end for that target, so the car kept the engine running to charge it up. Then the charge was high enough, so it stopped doing that.

The only time HV mode will charge the battery beyond the set parameter, is when coasting down a mountain.