This is very frustrating since many small businesses are losing several thousands of dollars due to Shopify not offering support and solutions for the "cart-to-checkout" issue.
Shopify claims it is "not a known issue" implying, to me anyway, that they have no one working on it. When we have called within the last several weeks we are told "sorry this isn't happening when I (shopify rep) checkout on my phone/ desktop; maybe try clearing your caches"
It is not a cache issue. It is something our customer experience often. There is no number to contact Shopify, nor do you have an account representative even if you're running a $1M in sales annually.