When copying a multi part mask from one image to another, the mask doesn't copy properly

1/ choose an image with a multi-part mask and enter develop mode

2/ choose Edit / Copy to display the copy dialog

3/ select only the multi-part mask and press the copy button

4/ go to another image (actually a different but identical image)

5/ choose Edit / Paste to paste in the mask

6/ some portions of the mask are correctly positioned but others are in the wrong location