I just installed 21.0.2 and I'm having the same issue.
1) start a new document.
2) create a small ellipse in the center of the document.
3) create a group and move the ellipse layer to the group.
4) move the layer mask of the ellipse shape to the group (Cmd + Shift, then click drag the layer preview image onto the group)
The ellipse is now masking the group which contains a fill layer, however instead of the fill layer going to the edges of the canvas, it stops at the edges of the ellipse.
Side note: Why does having the path operator set to Combine Shapes "punch out" the group? I would expect Subtract Front Shape to do that, but I get the opposite expected result. Also I noticed when playing around with the Fill layer left over from the ellipse that it sometimes won't even show up when moved around the layer stack.
Switching to Legacy Compositing is still a workaround. But the path operators are still behaving in the opposite manner of what one would expect.
Note: This conversation was created from a reply on: Photoshop CC 2020: Layer Effects on Groups Clipping Error.