Maybe you inadvertently pressed Ctrl+SpaceBar which reverts font settings to those of the underlying paragraph style. If the formatting changes occur when you press Enter, a different style may be applied by the "Style for following paragraph" setting in the current paragraph style.

How was the document initially formatted? If someone just selected text and applied formatting, then it's so-called direct formatting (not stored in a style), and the style formatting may resurface, as explained above.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional