When a paragraph doesn't have applied a style, the Normal style should be automatically applied. Currently, similar paragraph obtains its style from the parent.

This is reproducible when exporting to PDF and RTF.

Workaround: Check if there is a paragraph without style ID and if so, set it to Normal:

foreach (var paragraph in this.document.EnumerateChildrenOfType<Paragraph>())
{
    var value = paragraph.StyleId;
    if (string.IsNullOrEmpty(value))
    {
        paragraph.StyleId = "Normal";
    }
}

Available in LIB Version 2017.2.731.