Hi,
I think it's because of the way the tab system you're using works.
With characteristics, HikaShop will have a hidden HTML area for the value of the custom field for each variant.
This way, you can actually configure a different value in the custom field in each variant and on the frontend, when the customer changes the selection of the variant, HikaShop will update the HTML of the custom field, with the HTML of the corresponding variant.
If the tabs don't work when the variant is changed, it means that way the tab system works, it doesn't do anything once the javascript of HikaShop is refreshing the custom field area with the HTML of the custom field for the new variant.
Supposing you have the same HTML for all the variants, what you can do is to edit the view file product / show via the menu Display>Views and change the code:
if(!empty($this->fields)) {
to:
this will remove the display of the hidden area for the custom fields of each variant. Then, the variant refresh system won't find it when the variant selection is changed and it will keep the main display as is, and thus the tabs initialized by your extension making these tabs will keep working even after that.
Another solution would be for the tab extension to add a bit of javascript code in order to register the hkContentChanged event fired by HikaShop when a new variant is being selected in order to reprocess the HTML on the page.
Something like this:
<script>
(function(){
if(window.Oby) {
window.Oby.registerAjax("hkContentChanged",function(params){
// reprocess tabs on the page
});
}
})();
</script>