If you import a rtf document, it produces a span instances with highlight color property set to ARGB(255,0,0,0).

Workaround:

foreach (Span span in document.EnumerateChildrenOfType<Span>())
{
    if (span.HighlightColor == Color.FromArgb(255, 0, 0, 0))
    {
        span.HighlightColor = Color.Transparent;
    }
}