Petrified Lichen Guard not give deadly poison.
As far as the proc itself, it procs on EVERY block, and the DoT does 12 damage every 3 seconds, stacking up to 5 times. This is effected by one handed weapon spec, and does 63 damage per tick on a fully stacked DoT with 5/5.
Give enemy deadly poison
Try to block with the item and no deadly poison on target
Thank you for the report, but this issue has been reported already in #4575