ClipToBounds is clipping to the rectangular, screen-aligned bounds of an element, not to the precise shape. I don’t know what made you expect otherwise, but this is what it was always indented to do.
We don’t have general masking support at the moment. We do have the Mask-effect, which allows you to mask some element with an image, and sometimes people can get mask-like behavior by using ImageFills on Shapes.
Support for a bit more general masking (element to element masking) is something we’ve been playing around with internally, but we’re not quite there yet, unfortunately.