I am using a storefront child theme, but we offer several T-shirts in 6 sizes and 5-6 colors so I had this issue with my site as well.
Our fix was to set the default form values for each attribute in the product variation section… Before this was set, it seems that the customer’s variation choices were “blanking” or setting the variation properties to NULL. After setting a default when the customer selects the size and color they wanted (instead of the default) everything works as expected.
Hope this helps!
