Splintered Harmony and Forgotten Melodies theory
I will die on this hill: TOG was based on a second chance timeline. The first timeline failed, and the board was reset / the fabric of time rewoven after someone (probably the King) bargained with the Stygain spiders.
So at the end of KOA whilst the kingdom might have been saved (unlike the first timeline) - perhaps it was without realising that pulling tight on one side of the web to fix a broken loop, creates immense tension and structural fractures everywhere else. Maybe it fractured threads in other worlds within the Massverse i.e creating a “splintered harmony". And perhaps “forgotten melodies” are from past / original timelines that were reset?
A world doesn't exist in a vacuum; if you yank a single thread in the grand tapestry of the Maasverse, the neighboring threads in Prythian are bound to snap, fray, or splinter. Perhaps this explains why the harmony is broken - the "fix" for one reality became the fracture for another.