I would be looking at the document compatibility and layout options. If you go to File > Options > Advanced, scroll to bottom and verify both docs have exactly the same Layout options and Compatibility options.

The compatibility options won't be visible unless the document is saved in compatibility mode but there are settings there that do exhibit the spacing behaviour you are demonstrating. eg Suppress extra line spacing the way WordPerfect 5.x does

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Andrew Lockton
Chrysalis Design, Melbourne Australia