I like to think the First Order Stormtroopers were the ones who leaked the story.

Consider the following. The Supreme Leader is dead and Kylo is the new boss. All eyes are on him in the first battle - it will determine how the troopers view him behind his back.

And what does Ren do? He makes an absolute fool of himself in front of his entire army. Maybe he can fight, but now everyone knows he’s a terrible tactician, not noticing the obvious signs that Luke wasn’t really there.

Perhaps a couple companies were inspired and deserted shortly after, like shown in TROS, and explained the story to the galaxy at large.