I've put my PHEV in HEV mode and observed the electric miles drop by a substantial amount when the car needed to "borrow" from the battery, such as when driving at high speed on mountain highways. But in my experience, the ICE always pays back what it borrows, given the opportunity. For example, maybe you have 16 miles of EV range and you put the car in HEV mode. I've observed that when driving on a flat highway around 70 oor 75 MPH, the EV range might drop to 13 or 14 miles and then several minutes later it might have increased to maybe 17 miles. It usually stays pretty close on a flat highway, but when climbing a mountain in HEV mode, I've lost more than 10 miles of electric range. On one occasion, that EV range came back later in my trip. On the other occasion, I had maybe 7 miles of EV range left when I reached the top of the mountain, I switched from HEV mode to EV mode, and was surprised that over the next 16 miles of flat driving (mostly at highway speed), my EV range didn't drop at all and still showed about 5 miles when I reached my destination (my recollection of the exact numbers might be slightly incorrect - this experience was a few months ago). I don't think the ICE was running after I switched back to EV and I've never understood what the explanation was for being able to drive so many more miles in EV mode than the EV range indicated I had available after I reached the top of the mountain.
But my impression is that in general, the car will borrow from the EV range when it needs power and the PHEV battery has power to spare, and then it will try to pay it back to whatever range you had when you manually switched to HEV mode when it has an opportunity to do that. It usually doesn't seem to charge beyond whatever the range was when I switched by more than one or two miles.
2018 Niro PHEV Gravity Blue