Noticed this 3 nights ago on Blood Princes. I was looking to capitalize on ISF proccing while hard-casting Soulfire. It did and I got no additional Soulfire after my hard-cast was done. This happened several times throughout the fight so I just wait for ISF now when mobs are sub 80% HP and Soul Burn is on CD. Above 80%, I weave Soulfire into my priorities to keep the ISF Buff active.
offtopic: On blood princes, it's nice to shoot soulfires at the kinetic bombs to keep improved soulfire up /