TheManFromCannes mentioned in the comments a couple of days ago that Morfydd Clark shows her butt twice in Saint Maud. Here are some more details. The first scene, where Morfydd is having sex cowgirl style, is around 45 minutes in. Morfydd’s really riding the guy hard but her ass is partially obscured by the guy’s leg. A few minutes later, Morfydd is shown in the shower briefly. Part of Morfydd’s face can be seen in both scenes.