Please try to activate Storefront theme – official WC theme and check if you have this issue with our theme only or even with official theme too. In case, the problem appears only with our theme provide link to your variable product. Also, try to disable all 3-rd party plugins and then check.
I have tried what you have suggested but the problem remains.
I have also tried to upload a a backup of the website of 2 week time prior to the the date that the problem appeared, but even with this , all the attributes are gone.
We’ll be able to help you only in case problem appears with our theme only and works fine with Storefront.
It means that not our theme is a problem. Check your settings or contact WooCommerce support of WC support forum.