Found this on 1.16.1. Reproduced it on 1.16.2 World and screen shot attached.
Load the world. Observe the minecart appears to be on the hill, but stuck and not moving forward. This state will randomly pop up even if the cart is happily looping on the track. It just stops right there on the hill. You can manually push the minecart to get it to loop on the track, but sometimes it just stops like it is in the saved file.
Replacing the flat powered rail with plain unpowered rail does not seem to change the behavior. (happens both ways)
A key to reproducing this may be how much velocity the cart has before it hits the unpowered rail, controlling where between the hill and the flat it stops while unpowered. When power is reapplied it does not continue.
FYI (T;DR) this is intended to be a simple redstone pausing mechanism to allow minecarts a brief amount of time to collect from hoppers overhead. The small hill is intended to cause the minecart to continue forward when power is re-applied to the track. The powered rail is powered by default and only off for a short period of time while the cart is looping.
I found an ancient bug that might be related: MC-100954 or this may be an issue with the transition from sloped to flat track?
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Could reproduce in 1.18.1, but what I encountered duplicates MC-8402
I attached another screenshot with my setup, the steps I took to reproduce are:
Build a minecart track with the structure in my brightest (day-time) screenshot present
Place a minecart in the unpowered rail and gently push it until it's somewhere in between the two rails, but still on the slope as shown in the both the original and my darkest (taken at night) screenshot
Power the rails, if you set this up correctly, the minecart won't be activated, if not, you'll have to go back to step 2
I believe this is caused by the minecart technically being in the middle of the two rails, receiving power from none of them.
The notes on MC-8402 suggest that if the cart is on a slope, that it would coast to induce movement and then be accelerated in the direction of the movement. Perhaps a different way to reframe this is that the visual appearance in the image shows the cart on a slope, but it does not coast downhill when the brakes "are removed" (i.e. power applied)? A problem of friction and gravity, not of power to the cart?
I am unable to reproduce this issue in 1.17 Release Candidate 1.