Wrong, it used to (classic) happen with the application of SW:P and Mind Flay (not subsequent damage).
However, there was some lazy developer coding used because it was abused to despawn all mobs on Razorgore which probably broke it for SW:P and Mind Flay.
Don’t think it’s ever procced off SWP.(Maybe classic?)
Don’t think mind flay is considered a direct damage spell either?
The description is " Chance on landing a direct damage spell to deal 100 Shadow damage and restore 100 mana to you. (Proc chance: 10%)](
Direct damage spell.