[Quoth]

I forget how Word works, but you can search and replace styles in LO Writer. Styles in LO Writer* or MS Word map one-to-one with CSS classes when docx is converted to epub.

So I KNOW these spurious classes are spurious paragraph styles or direct formatting in the docx.

Of course direct formatting is very bad and harder to get rid off. Simply resetting format to paragraph style of identical as big as possible chunks of a chapter works.

[* Only edit LO Writer in odt format saves/loads after initial docx import, then for epub do an extra Save As in docx also. This is because otherwise if you edit docx it's a conversion every time with multiple similar page, paragraph, heading styles etc that need fixed each time the docx is opened. This is also an issue exchanging docx between different versions of MS Word!]