Hi,
There is no mechanism to hide / display custom item fields based on the selected variant.
So when you create custom item fields, they will always display on the product regardless of the variant selected.
If you don't see a custom field, it means that either you turned off its display option for the area where you're trying to display it or you've restricted it to another product / category / user group and that's why you don't see it.
So if you don't see it while you want, it's probably an issue with the settings of the custom field itself.
Could you provide a screenshot of its settings please ?