Edit: disregard, I was using activator rails instead of powered rails!!

It seems that when I place a minecart on top of a closed loop system of rails with a powered rail that is on, it doesn't power the minecart. Even after manually pushing the cart it eventually stops even shortly after passing over the  powered rail. Tested in open and enclosed spaces, a large and small loop, and with more than one continuous powered rail, the result is the same. Using a cart without a hopper, the powered rail acts like it's trying to break the cart instead.

Edit: I have now also tested the above with detector rails beforehand, the powered rail lights up but still do not power the minecart.