What happens if you click in the problematic paragraph and press Ctrl+Q (which runs the ResetPara command)?
If Ctrl+Q doesn't make a difference, click in the first Heading 1 paragraph of the document and click Home > Multilevel List > Define New Multilevel List. Verify that settings are correct. See also
https://shaunakelly.com/word/numberi...10.html?p=1768.