Hi,
That's normal.
When a variant has no description, it takes automatically the description of the main product. If the main product also has no description, then the variant system doesn't change the description already displayed.
So enter some text in the main product. For example, you could enter just the HTML:
That will add a space in the description. That way the description won't be empty for the variant refresh system on the product page, but customers won't see anything for the variants using such description.