The Issue
Monolith Lagon-related prophecies were not progressing for one of my characters, even though the conditions were met (Death of Lagon, God of Storms, Ending the Storm Timeline (xx corruption)
Possible Cause
After not being able to advance these prophecies despite killing Lagon with 300+ more corruption than required and even exiting and relogging my character, I believe I have found the cause. My character (an ES VK) was able to effectively kill Lagon in stage 1, destroying both tentacles. The game still progressed the echo to stage 2 and then after killing the wave tentacles back to stage 3 where Lagon immediately slumped over dead and the Harbinger spawned.
By purposely not killing Lagon in stage 1 and letting the echo progress normally, I killed Lagon in stage 3 and then the Harbinger. All of the blocked prophecies immediately progressed.