Thanks for the report, and especially for testing Windows 11 as well, because that comparison made this much easier to pin down.
I can reproduce it here on macOS and I have found the cause. It affects more than the shaped window: since the feature shipped in 26.10.2, all three window transparency features on macOS (Opacity, BackgroundColour alpha, and Mask) are being knocked back to fully opaque shortly after the window opens, which is why you see the rectangle instead of the circle. Windows is not affected, which matches what you saw.
The short version: the desktop runtime re-applies its transparency request to the window right after showing it, and the macOS window backend treats a repeated identical request as "stop being transparent", so the transparency that was granted a moment earlier gets cancelled. The fix is to stop re-sending a request that is already in place.
I have logged it here: https://feedback.objo.dev/bug/1742
I will post an update once the fix ships.
