What would energy “escape” into?
The circular universe theory is essentially the same as the round Earth theory. Keep moving straight along the surface of the Earth and, eventually, you’ll end up exactly where you started. Travel east, and you’ll eventually end up in the west. You’re not moving in a straight line (otherwise, you’d eventually leave the planet).
But, there’s no such thing as a “straight” line. It only looks straight due to our hypermicro inspection of the curved line. Keep moving in what you perceive to be a “straight” line and eventually you’ll end up back where you started. Travel to the east on the Earth and you’ll come in from the west. Travel ‘straight’ out of our galaxy and, eventually, you’ll end up back in our galaxy.
Assuming that the galaxies don’t move, of course, which they do, but for the sake of argument…