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.

 

Anything that only happens sometimes with any car is hard to figure out. It's up to you to narrow down the what where when and how of the things that are common when you notice the behavior.
Here are a few clues of what to think about when it happens to your Prime.

What speed are you driving ? Does it always happen at the same mph?
What's the state of charge of the high voltage battery? Does it always happen at the same SOC?
What location when it happens? Does it always happen in one area?
What type of weather? Is it always the same weather pattern when it happens?
What is your Air Conditioning temp set at? And what temp is it outside?
How far have you driven when you notice it?
What are the traffic conditions at the time?
Does it always happen in heavy traffic situations where you need to pull to the right?
How many mph does the car actually slow down?
Does the Prime always slow down the same amount in mph?
And even things as simple as what time of day or night, as long as it's consistent every time.

If any of the items on the list above are always the same when it happens that will give some clues as to why it's happening.,
We'll all still be guessing at why it's happening, but at least we might be able to make some batter guesses/