I didn’t see anything in the patch notes about the strikedown attack in the mk IV claw sword’s light chain (light #3) being changed to a vanguard attack, and the attack descriptions in the item inspection still show is as a strikedown attack. However, it has indeed been changed to a vanguard attack, with lower damage and higher cleave. So, the bug is that either this change was accidental, or it’s that the patch notes and weapon combo icons weren’t changed to match.
I’d like to add that this is a bad change, that leaves the weapon with no strikedown attack in its light chain and severely weakens the light>heavy attack chain that players typically use against hordes with the Mk IV.