I have seen this, too, in documents created outside of Word. To fix the problem, do the following: Make sure that the affected document is the active window. Press Ctrl+Alt+Shift+S to display the Styles pane. Click the Options link. Word displays the Style Pane Options dialog box. Just clicking OK to verify the settings may suffice, but (just in case) you can change some settings in the dialog box. For example, choose something else in the "Select styles to show" drop down or change the option for sort. When you are done, click OK.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional