This is sort of my fault. I accidentally found my way in to the Sorrow Heart before I completed Breaking the Seals. You can pull yourself over the gate barrier with your climbing picks. I got through the tower and killed the Bound Sentinel, and have gone on to run quests in Hollowed Moor.
Now, after circling back, I remembered I had not yet broken the second Sorrow Seal. I broke it and returned to the Sorrow Heart. Now my quest says to go kill the Sentinel. Since I killed it already, I can't trigger the point as completed.
Any work around?
I ran into this issue as well. You can fix it by resetting the realm and doing everything over again (though the seals remain broken so you don't have to repeat those).