The most likely issue here is HV battery temperature. The more you use the battery the warmer it will operate. The battery will not discharge below a pre determined threshold so the highway is not charging it is most likely cooling. Above and below specific temperatures the hybrid will limit battery usage and this can affect vehicle performance.
First check for HV cooling obstructions like floor mats blankets or "fuzz bunnies" in the HV battery cooling air intake. It is on driver's side under the big plastic carpeted cover. If no obstructions on the expanded metal intake continue to below.
When this happens again try the following for 15 to 25 minutes.
All windows and or sun roof closed. HVAC on high blower front and rear. Vent/Duct setting to floor. Set temperature to 60F. Max Cold NO recirc.
Drive above 45 mph
for about 15 minutes. Why?
HV battery cooling air comes from the long slots in the HV battery cover just above the floor under second row seat. HVAC set to floor will cause a pool of colder air to form on the floor just in front of these inlet slots. You can feel it with your hand in a few minutes after these settings.
Inlets must be unobstructed!
The floor setting also pulls in outside air. Recirc is colder but the outside air actually pressurizes the cabin increasing cooling flow through the HV battery.
Higher vehicle speeds with outside HVAC air creates an even higher positive cabin pressure.
So steady state highway speed for 15 t o25 minutes will raise the charge but the temperature is a greater effect.