A deliberately opaque corner of Japanese security policy is this country’s relationship with Taiwan. Taiwan is vital to Japanese national security and, to put it delicately, a government in Taipei that makes its own decisions well serves Tokyo (as long as it doesn’t go too far).
At the same time, however, Japan has long sought a positive, cooperative relationship with China; and support for Taipei makes that much more difficult.
China-Taiwan relations have deteriorated in recent years and Trump administration policies have magnified tensions. Frictions are unlikely to diminish in a Biden administration.