It's not necessarily that strange. If (for example) she's still getting tired easily and/or has an injury niggle, it might well make sense to miss out on Beijing (with an incredibly strong qualifying field and not many points on offer unless you make the main draw and win a match) and stay in Japan to get ready for her title defence in Osaka.

She is in Beijing( per Instagram), but must have withdrawn whilst preparing, strange, but as Steven says must be something not quite right.

Cheers Phil.