Whenever I try to export product variants, I got a weight or product name field in each variant field. This does not occur in your demo. I have tried changing various template settings or fields but to no avail. The result is the same every time.
In the screenshots, custom fields are are in czech (Barva, Tvrdost, Váha, etc.)
Using
RO CSVI 8.9.3
Virtuemart 3.6.10
Joomla 3.9.20