I feel like this is one of the larger things holding East Asia back from ever nearing reconciliation. "Salty" doesn't really describe it. I know it's an inhumane thing to say, but Nazi Germany getting ground down and pounded slowly back into Berlin and getting crushed between the Allies and the Warsaw Pact, as well as the post-war apologies and acknowledgements must have helped a lot in restoring their relations.
Today's Germany hardly has to deal with Nazi accusations in politics(memes about EU being the new Reich don't count). The nations Germany attacked had closure of some extent, getting to reclaim their homelands and drive out the invaders. Watching the enemy get split up and turned into a servant state must have felt like justice, however cold-hearted.
From the point of view of the people of China and Korea, Japan invaded, subjugated, looted and pillaged and after you spend long years fighting an increasingly desperate war of resistance, and even begin to recoup some losses, it's over. Two nukes were dropped and they've surrendered to the US. You never got to pay them back for what they did, never set foot on the Japanese home islands and brought the fight to their cities the same way they did to you, and you even had to watch the post-war Japanese economy get flooded with aid because they're now America's best buddy in East Asia to fight communism. Even today Japan remains the "one good Asian country, so civilized, so clean" that Westerners fawn over, despite having never had to pay the same way Germany paid.
It's not hard to understand how absurdly sour the grapes must be.