There appears to be a bug in Copy and Pasting a mask. It is difficult to describe for an abstract (general) case, so consider the following specific example. Start with an image layer and apply a mask. The mask is presently entirely white and the size of the image. Make a circular selection on the mask and fill it with black. The mask remains the size of the image and is now entirely white except for the "hole" you created. Copy the mask. Paste the mask onto another image layer or color adjustment layer. Paste the mask. The result is a mask of the size (in effect) of a rectangle bounding the black area of the original mask, so in my example, a rectangle (square) bounding the circular hole. In the original mask, the entire area outside the circle is white. In the copied and pasted mask, only the area between the circle and its bounding square are white. The mask is black outside that bounding rectangle.
Conversely, if the original mask is copied by option-dragging it (in the layers list) to a new layer, the new mask is, indeed, a copy of the original.
Mac Studio Ultra
macOS 13.3
Pixelmator Pro 3.3.2
Copy and Paste Mask Bug?
2023-04-06 18:00:17