I tried to make an example to make sense, here try this, you're coming to a toll booth, so you switch to EV now to recharge while braking and then go back to EV later as you pull away from the toll booth. Use this analogy in the rest of the example and see if that makes sense to you.
It can be deceptive to use only the percentage indicator in the MFT, because it can go above the reserve when it is the ICE charging and the indicators are white (ICE on) instead of blue (EV).
If doesn't make ANY difference where the charge comes from, the engine or the hill or stopping for a toll booth. If you made extra charge over 55% threshold set in EV later, cycling EV later will bank that extra charge %. Doesn't matter where you made it, you still lose MPG after you cycle because the engine has to recharge higher now. +1 to +2% more before leveling off.
So, if you go down a hill and get +2% more and you bank it, well the engine now at 57% has to charge to 59% before you start getting good mpg numbers. If you didn't bank it and the threshold is still at 55% and you're at 57%, then you're going to benefit immediately from better mpg numbers as at 57% the engine doesn't have to charge the HVB any more.
There are many variables into play, you need to watch everything. From vehicle speed to road grade to power demand to charge % for all things to fall into play and determine your mpg numbers.