Manchu people's loyalty nowadays, at least technically, still lies to the Kingly House of Aisin-Gioro (Qing). The later just happened to be Vassal of the Japanese Emperor, and Japanese investment and economic development did helped them going from one of the poorest region of China into one of the richest developed parts of the World.1) Manchuria remaining part of Japan was not entirely a case of Japan simply refusing to give China something back. A vast majority in the province much preferred the rule of foreigners who at least were somewhat in touch with them to Chiang's corruption and despotism. Indeed, this fact was cited by the UK early on in the Cold War.
You just don't bite the benevolent hand that led and fed you, that's why Anglo-Japanese bloc thrives despite the best American effort to undermine them.