thinking about nanakibh's videos on the master of masters and the theory that the master of masters created so many rumors/interpretations of himself in order to make himself disappear, and how no one in the series seems to know what kingdom hearts is.
it's described as "one great light that protects us all", that "ancient people believed that light was a gift from an unseen land by the name kingdom hearts", no one wrote about the color of it, no one wrote about what the door to it or key is to look like, there are multiple theories on how to create the door, it's said that once someone enters the door they'll be "reborn as something far greater than human" but also that one "[gains] control over all worlds and all people" but again it was "an unseen land" so how would they know, and so on and so forth...
so what if the real reason kingdom hearts disappeared after the war wasn't just because darkness was introduced, but because of the misconceptions and ambiguity surrounding it? the rumors and theories surrounding kingdom hearts grew to such lengths that a war broke out... much like with the master. the amount of confusion around why they were collecting light, what the master was like as a person, what he even looked like, who the master distrusted, who was his favorite, what the master's true goal was, his abilities, his intentions, what he wanted from each individual person, and how much knowledge he actually had about any given situation led to war. a fight over who deserved power and attention from MoM. in both instances, be it god-like power or a reward for placing high on MoM's leader board society, kingdom hearts and MoM had mystery and potential power to give and war caused them to disappear from that world.