I seem to recall years ago problems with section break types changing by themselves. Doing a google search reveals line numbering could be a culprit eg https://answers.microsoft.com/en-us/...e-1ad471d9de39

Can you change the section break type after it changes? Put your cursor somewhere in the section after the section break and go to the Page Setup dialog > Layout > Section Start = Continuous

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Andrew Lockton
Chrysalis Design, Melbourne Australia