2017 Prime Advanced - Drop in acceleration after switching to gas

I think I would look at the common things ICE related.
Bad/old gas.
Gas filter dirty.
How many miles on the beast?
If you have a lot of miles maybe a set of plugs is in order?
Injectors need cleaning?
You get the idea.
I wonder if it's throwing any codes that might give a clue as to what's going on.

 

if the battery happens to be low at that point, you won't get much out of the little engine, or it's hot out, the battery will be protected by the software, limiting discharge rate.

 

That's common on long steep inclines, such as the Altamont pass. The engine is cold and the ECU won't dedicate all engine power to the axles until the engine warms slightly. Whenever I'm heading into the mountains, I manually switch to HV mode, when I get on the highway, to make sure the engine is warm when I hit those slopes. You would reserve some extra traction battery for the climb and the engine will be warm, so there's less or no drop-off in power.
If this isn't it, have someone with a Toyota Techstream scanner check the ECU history for a pending P0300 code - intermittent miss fire. You may have an intermittent faulty spark plug or ignition coil. When your at high rpms, the ignition coil can't keep up and will start missing. Drop the RPMs down, and all the coils can keep up, so you get your power back.

Those are my guesses for the day - Hope this helps.