Hello,
This kind of problem is classic with the dropdown display, as you can imagine when your customer arrived on the product page, HikaShop can't anticipated product variants configuration, and so the dropdown list all variants, in order to remove a variants because a first variant is selected, HikaShop will have to refresh the product page, and the dropdown with it.
When you have variants that cross each other, the best way to display this will be the Table mode :
Hope this will fit your needs.
Regards