domain mapping + woocommerce problem

Identifying this as a domain mapping problem, but could be WooCommerce as well. This scenario worked fine for months and has stopped for an unknown reason.

Main site:
Domain mapped site:

I have one specific page set to be excluded from domain mapping:
That is due to SSL only functioning on sub-domains, not the domain mapped site.

But now when clicking checkout, the page refreshes and goes back to a now empty cart. Interesting... just noticed that it redirects to which should technically be impossible because it should be reverting back to the mapped domain!

Support access enabled.