Hello,
So, we have further checked this concern on your Site and deactivated all the third party Plugins on your Site and then tried creating Custom Layout for Single Product, it works fine by then.
So, it is a third party Plugin conflict on your Site in this regard not an issue at the Theme end.
You now need to reactivate them one by one to see which Plugin is causing the conflict.
We have created the new Layout on your Site for the Single Product named as “NEW SINGLE”.
Please check back your Site after completely clearing the browser and server cache.
Note: We have remained the third party Plugins deactivated on your Site, so that you can check the rectification first.
Best Regards.