Created attachment 444363 [details] demo of the bug Having a box-shadow with negative spread and inset displays the shadow as solid color and ignores the alpha from rgba color of the shadow and the blur. https://codepen.io/zlatip/pen/eYEQeaB
I cannot reproduce this with macOS 12.1 beta.
Notice the OS version and safari version I have set. It is fixed indeed in latest Safari (15)
Thank you for testing.