Card headers and footers maintain border radius inside a card group

[onokazu]

The issue with card header/footer border radius of cards in a card group is still present in Beta 2 although the issue initially raised (#19159) has already been closed.

Try zooming into the footer section of the 2nd card-group example on the documentation site. https://getbootstrap.com/docs/4.0/components/card/#card-groups