巴黎和会上日方代表已经同意交还山东的权益

有兴趣的可以看Paris 1919: Six Months That Changed the World这本书，里面一章就是讲中日在山东的问题的。

这本书里说，日本在会后就找中国制定解决方案，最后在华盛顿会议上解决的

The Japanese government, which had not counted on the depth of opposition, began to think that it should keep the promise it had made in Paris to hand back its concessions in Shantung. At the beginning of 1920, it tried to open negotiations with the Chinese government to withdraw Japanese troops from the province. The Chinese declined to discuss the matter. In the autumn of 1921, Japan made a renewed effort; it suggested conditions under which it could give up its rights in Shantung. The Chinese government refused to give a clear answer.

Finally, at the Washington naval disarmament conference, with the British and the Americans acting as mediators, Japan got China to agree to a settlement under which China resumed full sovereignty in Shantung on February 4, 1922. The railway from the port of Tsingtao to the interior, which had caused such trouble, was sold back to China under a complicated scheme that effectively left Japan in control for the next decade. China was probably the loser in financial terms: the railway, as the Japanese had discovered, was unprofitable.54 In Washington in 1922, Japan also signed a treaty with the other powers guaranteeing China’s sovereignty and territorial independence. That guarantee ran out in 1937, when Japan invaded the mainland of China, and Shantung, along with all the coastal provinces right down to the south, passed under Japanese control.