Hello,
I have logged into the staging website and checked by activating the Storefront theme and the issue with the SKU is the same.
Screenshot: https://gyazo.com/057e04d3dcc8c6505f25e64a8c9a0dc5
Which clears that the issue is not from our theme. Sorry but we could not help you with this as we could not help with third party issues. You can check after deactivating all the third party plugins.
Best Regards
