The \sl (line spacing) RTF tag is supposed to be accompanied by a number indicating the value of the line spacing. According to the RTF specification: 

Space between lines. If this control word is missing or if \sl0 is used, the line spacing is automatically determined by the tallest character in the line. If N is a positive value, this size is used only if it is taller than the tallest character (otherwise, the tallest character is used); if N is a negative value, the absolute value of N is used, even if it is shorter than the tallest character. 


However, there are documents where the number is missing at all and MS Word interprets those as if the value is 0. RadRichTextBox visualizes these with lines of size close to zero which makes them invisible. Although this case is not mentioned in the specification, we can consider fixing it.