line spacing changes itself

Hi, I have a ten page .doc document (Win 10/64, LO, set to “Single” line spacing. When I reopen that doc, all line spacing has changed to "Proportional of 115%. Why is that?

Does this happen if you save in [.ods] correction .odt format? If so, just save as a .doc when you finish the document; note that recent versions of MS Office (since around 2007) can read ODF files.

That document was designed in Writer, Word does not apply here.
When I set the spacing to single, shouldn’t it stay that way?

I understand that you are using Writer; but your question implies that you are saving in MS Office .doc forma rather than the native .odt format. “I have a ten page .doc document”

You may lose some formatting if you save in MS Office format. See [Tutorial] Differences between Microsoft and AOO/LO files (View topic) • Apache OpenOffice Community Forum