Boss using heavy attack with no audio/visual indicator ID: #226677994

Just happened on a T15 map with 3x Ire.

Cleared the whole map, got to the boss and the first few times he attacked, you had the audio + visual indicator that a heavy attack was coming but then he stood in a corner after a charge not doing anything for a few seconds (not moving or doing literally anthing) after which the heavy attack just 'appeared' in front of him killing me instantly.