The Grapevine is a pretty intense drive for any car. You're getting roughly 50% of the rated EPA range/MPG which is what I experience in an ICE car going through the Grapevine.
For those unaware this is a stretch of road that through several sections ascends several thousand feet of elevation with a median traffic speed of about 70 MPH. It's so severe that there are signs all along the road to turn off air conditioning to avoid overheating.
The back side of the Grapevine drops in elevation so much I could usually pick up 10 miles of EV range in my Volt.
Depending on the section of the Grapevine this is not abnormal at all. This is one of the roads that "Mountain Mode" was designed for (literally) and I would recommend using it to avoid loss of propulsion power and overheating.
Agree... in fact the Grapevine is so intense in spots, big rigs must stay in the truck lane.