They usually curl up on my thigh. I think they only work well for very thin women unless the leg is quite long. I have some boy shirt bikinis that worked fine. I guess it was the fabric, and I was thinner when I owned those bikinis so I’m not sure if I could wear a bathing suit like that now.
I do have a couple of pairs of 100% cotton from Jockey that the leg stays down, but they are a little large on me, and terrible to wear under slacks or jeans that fit right against my body. I just wear those underwear at home with long t-shirts, that sort of thing.