Hello Siyu
I would like to suggest some steps that you can try to check this issue:
1/ Try opening Word/Excel in Safe Mode
Press Windows + R
Type: winword /safe (for Word) or excel /safe (for Excel)
If it works in Safe Mode, add‑ins may be causing the issue

2/Disable Overtype mode
- Launch your Word document, click on "File", then select "Options" from the fly-out menu.
- Click on "Advanced".
- Navigate to "Editing options" and uncheck the "Use the Insert key to control overtype mode" and the "Use overtype mode" boxes.
- Hit "OK" to save changes.

3/ Clear all text formatting
- Press the CTRL+ A combo to select all text on your Word document.
- Click on the "Clear all formatting" icon.

You can close the current Word document and try the same action in a new file to rule out the possibility that the issue is related to a specific document.
Alternatively, you can temporarily use Word for the web while continuing to troubleshoot the issue in the desktop app version.
If the issue still persists, please feel free to let me know so I can try to review it again for you.
Have a nice day!