Every size option has an associated option id mapped in the DB. Whenever options are uploaded/updated, the mapping changes. For example, Size S has option id 3, but when the options are updated/uploaded, it might happen that the option id is now changed to 12. Now when a order is created for Size S, system tries to look for option id 3 which now may be empty. In this case the order will be created without Size.
To solve this we have to ensure that every product has at least one size associated with it. For products like bags, jewelry, bed sheets etc, we should use OS as size.