Why did Stalin refuse to return the Chinese territory occupied by Tsarist Russia? Lenin even said he would return it

It is indeed narrow-minded great-power chauvinism.

In terms of relations with China, Lenin believed that the Chinese territory occupied by Tsarist Russia should be returned to China, and promised to give up all Chinese territory captured by Tsarist Russia and China Concession. But after Stalin came to power, the Soviet Union withdrew this promise. In 1921 and 1924, Tangnu Ulianghai and Outer Mongolia broke away from China, and the Soviet Union gradually took actual control of these two areas. In 1929, the Middle East Road Incident occurred between China and the Soviet Union, which led to the deterioration of Sino-Soviet relations. In 1931, the Japanese army invaded Northeast China and the Soviet Union remained neutral. After the formal war between China and Japan in 1937, China requested international assistance. The Soviet Union believed that its Far Eastern border was threatened by Japan. After the signing of the "Sino-Soviet Non-Aggression Treaty", the Soviet Union began to assist China on a large scale and dispatched aviation troops to participate in China's war against Japan. The Soviet Union became China's main aid source in the early days of the Anti-Japanese War. In 1938 and 1939, the Zhanggufeng incident and the Nomenkan incident broke out between the Soviet Union and Japan due to the Manchukuo border issue. The Soviet army completely wiped out the invading Japanese army. In order to avoid fighting on both sides, the Soviet Union signed the Soviet-Japanese Neutrality Treaty with Japan in 1941 when Soviet-German relations were very tense. In addition to stipulating mutual non-aggression, the treaty also recognized each other's actual interests in Outer Mongolia and Manchuria. The Soviet Union suspended most aid to China.