After conversion the DOCX should show up as a Format in the Book Details panel (right sidebar) if you click it then it should open up in whatever you have set as the DOCX handler (Word, Writer etc).
Or press Alt+V and select the DOCX.
Added: where is the library, if it is in a Controlled folder, such as Documents, you may be running into MS Defender's Ransomware protection feature, see ==>>
Enable or Disable Controlled Folder Access in Windows 10 | Tutorials
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