SOLVED:

Pretty much exactly what the title says.

Since the server update an hour ago I can heavy attack OUT of stance but once I focus on an opponent I can't heavy swing.
Instead RT now makes me switch who I'm focused on instead of heavy attack!

I'm playing on PC.

Thanks,

Rob

SOLVED - Issue was locking stance. Keep it locked instead of press and hold and then you can big swing, which is an annoying chance but oh well.