Hi @Quỳnh Nguyễn,
To narrow this down, does the font change in a new blank document as well, and does it happen with every character or only Vietnamese characters with diacritics? Also, are you typing with the Windows Vietnamese keyboard, UniKey, or the Japanese IME?
First, go to File > Options > Language and make sure Vietnamese is added under Office authoring languages and proofing. Then open a blank document and go to Review > Language > Set Proofing Language, select Vietnamese, and temporarily clear Detect language automatically. Word’s proofing language can follow the active Windows keyboard language.

If the issue continues, go to Home > Styles, right-click Normal > Modify, select Times New Roman, choose New documents based on this template, and click OK. Then close Word completely, reopen it, and create a new blank document to test the font.

Please give these steps a try, and if you notice any changes or have any updates, feel free to leave a reply and let me know the results. I'll be happy to continue assisting you and follow up further if needed.
Thank you for taking the time.