Hello,
Sorry to say but we have checked and confirmed this concern with one of the default WordPress Themes like Storefront (developed by WooCommerce) and the functionality remains the same there too so this isn’t an issue at the Theme level but how things by default works at WooCommerce.
On your Site, you can confirm it too by installing & activating the Storefront Theme once to confirm more.
So, to overcome this you can have the Stock Status enabled so that the Customers get to know already about the number of stock for the respective variant of a Product: https://snipboard.io/s4Kp3U.jpg
Hope this helps !
Best Regards