Xinjiang and Tibet have been part of China for many periods throughout history; Japan never was. At most, Korea was merely part of the tributary system. There is a fundamental difference here.

Tibet, too, was only part of the tributary system. Even during the Qing dynasty, the Chinese imperial state had no effective control over central Tibet - all local rulers and judges were Tibetan, and they employed Tibetan, not Chinese, law. Outside of diplomatic circles, Tibetans at the time weren't paying any attention to Chinese culture and politics.

Claims to the contrary are largely historical revisionism. (As are the various claims that Tibet was culturally influenced by China - the story of Princess Wencheng bringing agricultural technologies to uncultured Tibet, as it is often taught in Chinese schools and portrayed in period dramas, is a myth that only came to popularity during the Chinese Civil War.)

Remember also that until 1951, Tibet occupied Chinese territories more often than vice versa - although given the case of Manchuria, China might actually see this as an argument in favor of Tibet being Chinese.