Describe the bug

I upgraded to 2.0.0-rc.28 today and noticed the header-row stopped showing up on all my tables that had a stacked-option, e.g. stacked="md". I downgraded to rc.27 and everything worked fine.

Steps to reproduce the bug

See:
https://codesandbox.io/s/bootstrap-vue-table-bug-rc28-r31q3

Expected behavior

A header row is expectead as in the rc27-version

See:
https://codesandbox.io/s/bootstrap-vue-table-bug-rc27-xl2se