[luatex] Misplaced letter

[Javier Bezos]

I'm using TeXLive 2017. The following document renders the
text correctly.

==============
\documentclass{book}

\usepackage{fontspec}

\setmainfont[Script=Arabic]{FreeSerif}

\begin{document}

\pardir TRT
\textdir TRT

نشر لودجي جالفاني عام
\iffalse
   {\textdir TLT 1791}
\else
   1791
\fi
اكتشافه الخاص بالكهرباء الحيوية الذي أظهر أن الكهرباء هي الوسيط الذي
تقوم من خلاله الخلايا العصبية بنقل الإشارات

\end{document}
==============

However, if I change \iffalse -> \iftrue, the very last
alif is displaced to the left (a few points; this is not
the only misplaced letter). So, the local \textdir is
changing something (or the internal -TLT node). Actually,
I discovered this odd behavior with dir nodes inserted
with a lua script. Interestingly, if I comment \pardir TRT
out, the alif is at the right position (and in the complete
document, if I reset \pardir TLT).

I don't know where the problem is. A bug? I missed
something?

Javier