I would not be surprised if this was our old enemy, Basic Paragraph. The Basic Paragraph style was a terrible idea added several version ago. It is a paragraph style you cannot delete and should never use, never edit, and never base any styles on.
To understand this trap it helpos to understand how paragraph styles work, espcially between documents when using styles with the same name. Let us say in both Document A and Document B you have a stlye called Body Copy. In Document A Body Copy is 12 point TImes Roman. In Document B Body Copy is 11 point Helvetica. If you copy a paragraph styles as Body Copy from Document A to Document B you will select some 12 point Times Roman text, but when you paste it it will become 11 point Helvetica. The pasted text adopts the formatting of matching styles in the target document.
This is usally acceptable and is not likely a surprise. The problem is that every document has a style called Basic Paragraph. For most installations of InDesign this is the default format for any new text. That is, create a new document, add a text frame, and start typing. The text you type will be formatted as Basic Paragraph. Create a new paragraph style and it will be based on Basic Paragraph. Some people will see this style is in their new odcuments and will use it as the default style, editing the style. If that text is copied to a different document it will almost certainly be reformatted to match Basic Parahraph in the target document. This will happen even if you are not directly using Basic Paragraph. Say in Document A Body Copy is based on Basic Paragraph, but in Document B Body Copy is not based on any style. Even if the formatting for both Body Copies is the same pasing Body Copy from one document to another can cause reformatting because the Basic Paragraph styles in each document do not match.
In short: Do not ever use Basic Paragraph, do not ever edit it, and do not ever base other styles on Basic Paragraph. Also, vote for this… https://indesign.uservoice.com/forums/601021-adobe-indesign-feature-requests/suggestions/43146405-eliminate-basic-paragraph-style
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