Net/Gross Weight fields on Fixed Asset are part of Intrastat extension (both fields were already available on Items since the beginning). Net / gross weight fields on fixed asset card are not transferred to sales line / purchase line .

Basic problem : if you print the draft invoice (sales), then the weight is not printed.

If you sell an item, then the net/gross weight fields on item are transferred to sales line so on the draft invoice , the weight is printed.

Suggestion : please transfer the net/gross weight of Fixed Asset to sales line / purchase line (same logic as item)