I am attempting to set up an A/B mobile test using Target VEC. However, when I set the mobile URL in the Page Delivery settings (i.e. m.mywebsite.com), the VEC renders and defaults to the Desktop site. I can confirm that I have set up mobile tests like this in the past using Target VEC and am wondering if others have come across this recently/functionality of the VEC has changed for mobile sites.
I think there are no changes done in page delivery settings as we didn't faced any issue till now.
Can you check if the element/content you have edited in desktop site and trying to deliver it on mobile site is present in mobile site with same class or id.
In case you have selected Use element id / Use element Classes in VEC for your activity as shown in following screenshot.
As Target uses element Id or class or both to generate css selectors (which you can configure in site preference option in VEC as shown in above image) and deliver the modification using that element selector.
Let me know if this helps or if you find some other issue.