I have a ten-year-old Peace Lily that I have brought back from near-death experiences several times over the years. I recently repotted it, not in a larger pot but to give it fresh soil (the old soil was half terra cotta beads, and was draining too quickly and I needed to water it twice a week). Almost three weeks since repotting, it seems to be very happy except on one side the leaves are horizontal, not vertical. I made sure it was evenly watered and move it every few days so all sides get indirect sunlight (it never gets any direct sunlight, north-east facing window). Photo attached showing the droopy section on the bottom right (it's temporarily in the kitchen after being watered). What would make just one section droop?
Probably unintended root damage when you changed its soil; its entirely possible to have some root damage that affects part of a plant, but not the rest. Even so, check the backs of the leaves and stems, just to make sure there isn't an undetected insect problem.
If the droopy parts continue to droop or start to go brown, you could try cutting them back to soil level, which would give the roots time to recover without also trying to support topgrowth, and hopefully lead to new growth later on.