some Chinese charcaters in free text annotation don't show

[Wi11iam21]

when I add some Chinese characters in Free Text Annotation, some characters don't show in the page, like below, I input 5 characters and there are 3 showed, I think it's the font problem, maybe the En font doesn't support Chinese characeters, but how can I add some Chinese fonts in the software? Or is there any other ways to address this problem? Thanks ahead.

[GitHubRulesOK]

That is a weakness in MuPDF which is the core engine for PDF handling including Annotation.
it is covered by the OPEN issue #2255
It does not use the heavier weight "Rich text" annotation methods of Acrobat
If you were to use embedded fonts and XML then annotations would consume vast amounts of file space!
If you must add such text the best option on Windows is try MS Edge as a one page viewer app it can add HTML text without a problem.

I can call Edge with the file at current page from SumatraPDF to add rich text then a save in that single window will be seen in SumatraPDF.
HOWEVER Edge shows nothing whatsoever for that text and SumatraPDF shows 3 from 5 on the page !

To be seen in BOTH requires a more complex editor like Acrobat Pro/DC or Tracker/Foxit with Rich Text ability.