I think the loss of everyone's memories came from Kingdom Hearts. Here's why i think this. In a previous trailer, we are shown Ven in OC with MX and Ven doesn't recognize MX (this is the scene when MX tells Ven to give up in order to gain blah blah blah) and then MX summons an orb. That orb is seen in the Secret Ending, and if my theory is correct, contains Ven's memories, in which MX created the Kingdom Hearts we saw in the Secret Ending. Which leads to another part of my theory: MX is defeated and the Kingdom Hearts is destroyed, and since it was made of Ven's memories, they (the memories) become destroyed, and an effect is caused similar to that when Sora was asleep in CoM and everyone forgot about him.
Now i know that his theory is a little rocky, and might've been discussed by someone else, but comment nicely please
Now i know that his theory is a little rocky, and might've been discussed by someone else, but comment nicely please