I hope your wife didn't have the e-brake on
I rarely get above 75, and I was thinking I was paying a price for driving that fast, but on a recent long trip, I didn't see any difference in MPG reported by the trip computer for the first hour cruising between 65 and 70 and the second hour maintaining close to 75. Maybe it all falls apart north of 75 though.
I have noticed something similar to what you described: just because I toggle into HEV mode doesn't necessarily mean that the car will immediately stop using the battery for propulsion. It clearly has a mind of its own about this, but it kind of makes sense: even a pure hybrid switches back and forth between using the battery and using the ICE. So even if I did attempt to switch to EV mode with a full battery, probably the computer would still draw down the battery a bit before it started relying on the ICE.
One thing I continue to wonder about is whether there's any disadvantage to retaining a partial change in the PHEV battery during a long trip. I've previously described the advantages that I perceive, but conceivably, there might be a disadvantage in that it's easier to drive energy into a depleted battery than it is to drive the same amount of energy into a 2/3rds charged battery. So a light tap on the brakes here and there when the battery is depleted might effectively store more regenerated "juice" in the battery than the same tap will accomplish if the battery has a stronger opposing charge.
My driving experience suggests that what I'm doing is benefiting my overall fuel economy though, but my experience is still pretty limited at this point, and there is probably more to learn about the best way to optimize this.