Out of stock variants not disabling “Add to Cart” button

The issue is that on any variable product where there is only a single option (eg. size) and a variant is out of stock, it does not disable the add to cart button. It also doesn’t trigger the “Join Waitlist” button.

Steps to reproduce:

1. visit: https://delta.ar15discounts.com/products/armaspec-stealth-recoil-spring/
2. click a variant
3. witness that it doesn’t disable the ATC button

What makes this more confusing is that any variable product containing more than one option. For example if it has both size AND color options, then this issue is not present. You can check this page which shows it functioning as expected: https://delta.ar15discounts.com/products/aero-precision-atlas-r-one-free-float-m-lok-handguard/

I am attaching screenshots so you can see the following:
1. That I disabled all plugins (except woocommerce) and this issue is still happening.
2. That I confirmed that the inventory tracking options are correct for the product
3. I also made sure that the out of stock threshold in wc settings is at 0

You can find the screen shots here: https://www.evernote.com/l/Aku6wAOzZrFAALUdKsUcwfxrwK8AdLsn0kA

The page I need help with: [log in to see the link]