Does the paragraph actually change to Heading 1 style or is it just the formatting of text that changes? As you probably already know, you can use the Apply Styles pane (Ctrl+Shift+S) to learn the style name at the insertion point.

The next thing to check is the default formatting of the Normal style. You can do that by selecting any text and pressing Ctrl+SpaceBar (which runs the ResetChar command and clears font formatting not included in the current paragraph style).

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional