Interesting, I’ve found some other people with the same general issues:
I just confirmed the same issue happening on the default TwentyFifteen theme (meaning it’s not GeneratePress specific).
You may want to ask the WooCommerce people to test out the checkout on IE11 using TwentyFifteen and only WooCommerce activated.
Hopefully they’re able to figure it out.
Let me know what they say.