Among the Allies, there was no recognition of any legal transfer of Taiwan's territorial sovereignty to China in 1945, despite the repeated claims of the Chinese Nationalists.
Aide-Memoire: The Department of State to the Chinese Embassy, Washington
Date: November 21, 1946
Subject: status of Taiwanese persons in Japan
. . . . It should, however, be pointed out that from the legal standpoint the transfer of Taiwan's sovereignty remains to be formalized; assumably a treaty of cession will in due course be negotiated which will effect such transfer and which may contain provisions in regard to appropriate change in the national status of Taiwan's residents.
[ source: United States Department of State / Foreign relations of the United States, 1946. The Far East, Volume VIII (1946), page 359 ]
See -- FRUS page for full content
[Chinese version] https://www.twdefense.info/trust3/frusch-1946aq.htm