Strange line spacing effects in longtabu

Question

I got some strange effect with longtabu:

As you can see the line with λ seems to be closer to the above line than to the one below of it. I cant’t figure out why …

This is the code with which I produced the above picture:

\documentclass[
    fontsize=11pt,
]{scrbook}

\usepackage{xltxtra}

\usepackage{array}
\usepackage{tabu}
\usepackage{longtable}

\usepackage[osf]{libertine}

\begin{document}
    \begin{longtabu} to \textwidth {lX}
         \textbf{L} & Musica incantans sive poema exprimens musicae vires per illam juvene in insaniam
        acto et musici inde periculum. Authore Roberto South Art. Bac. ex Aede Christi. Oxonii, Typis
        Leon. Lichfield Academiae Typographi, Impensis Tho. Robinson. Anno 1655. \\ 
         \textbf{λ} & \textit{Überarbeitete, zweite Fassung des Gedichts zwischen 1655 und 1667.} \\ 
         \textbf{W} & Musica incantans sive poema exprimens musicae vires juvenem in insaniam adigentis et
        musici inde periculum. Authore Roberto South Art. Bac. ex Aede Christi. Oxonii, Typis W.\,H.,
        Impensis G. West. Anno Dom. 1667. \\ 
    \end{longtabu}%
\end{document}

The workaround is to put an invisible rule to the λ line to make it higher.

Answer 1

It seems that tabu forgets to put a \strut at the end of an X cell, so

\begin{longtabu} to \textwidth {lX<{\strut}}

should solve the problem.

Answer 2

Another solution is to to put

\tabulinesep=1.2mm

before the longtabu-environment.

\documentclass[
    fontsize=11pt,
]{scrbook}

\usepackage{xltxtra}

\usepackage{array, tabu, longtable}

\usepackage[osf]{libertine}

\begin{document}
\tabulinesep=1.3mm
    \begin{longtabu} to \textwidth {lX}
         \textbf{L} & Musica incantans sive poema exprimens musicae vires per illam juvene in insaniam
        acto et musici inde periculum. Authore Roberto South Art. Bac. ex Aede Christi. Oxonii, Typis
        Leon. Lichfield Academiae Typographi, Impensis Tho. Robinson. Anno 1655. \\ 
         \textbf{λ} & \textit{Überarbeitete, zweite Fassung des Gedichts zwischen 1655 und 1667.} \\ 
         \textbf{W} & Musica incantans sive poema exprimens musicae vires juvenem in insaniam adigentis et
        musici inde periculum. Authore Roberto South Art. Bac. ex Aede Christi. Oxonii, Typis W.\,H.,
        Impensis G. West. Anno Dom. 1667. \\ 
    \end{longtabu}

\end{document}