Curvilinear elements can cause problems in the case of complex geometry and to find the nearest true surface might not be unique. To deal with this issue, it is recommended to increase the number of segmentation of the curvilinear element and it will work fine. One advice: at the same time, decrease Aspect ratio. I used aspect ratio=3. It results in a better quality mesh. Usually, Eigenmode problems are very sensitive to the mesh quality. My setting was 10 degrees with aspect ratio = 3