Hello,
Point is that single product sidebar is inside the row with the product image http://prntscr.com/pbv35i but related products, tabs are out of that row, so sidebar can’t be moved our of the row on the mobile version. To implement this we should move tabs to block with the product image but in this case, you’ll get the gap at the side of the tabs, related products http://prntscr.com/pbv4xb Is it suitable for you?
Regards