Open Virtuemart -> Custom Fields
Choose a 'custom group' from the filter dropdown.
Save the ordering
It forgets the selected 'custom group' filter
Also if all of the fields have 0 as ordering it keeps these values Bu when you list the products of a category and save the order it sets an incremental value for ordering. Could you please apply this method for customfields too?
Tested with VirtueMart 3.6.11 10251