[luatex] Problem with babel-greek (harfbuzz?)

[Nikos Platis]

Dear all,

I came across a rather strange issue while using lualatex with babel-greek.
I am posting here because I believe it is mostly related to lua(la)tex and
less to babel-greek.

Here is an MWE:

----------------------------
% !TeX program = lualatex

\documentclass{book}

\usepackage[sf,bf]{titlesec}

\usepackage[greek]{babel}
\babelfont[greek]{rm}{Times New Roman}
\babelfont[greek]{sf}{Arial}

% \usepackage{polyglossia}
% \setmainlanguage{greek}
% \newfontfamily\greekfont[Script=Greek]{Times New Roman}
% \newfontfamily\greekfontsf[Script=Greek]{Arial}

% \usepackage[no-math]{fontspec}
% \defaultfontfeatures{Ligatures=TeX}
% \setmainfont{Times New Roman}
% \setsansfont{Arial}
% \usepackage{xgreek}

\begin{document}

\tableofcontents

\chapter{Πρώτο Κεφάλαιο}
Καλησπέρα \TeX. Πρώτο Κεφάλαιο.

\end{document}
----------------------------

I was compiling this on fully updated Arch Linux with lualatex from texlive
(not Arch Linux packages). The result is the attached pdf file. You can see
the strange accents in "Περιεχόμενα" and "Κεφάλαιο". You can also see that,
for example, "Κεφάλαιο" appears correctly in the text.

After a lot of testing, I found out that this happens only for lualatex
(not xelatex) with babel-greek (not polyglossia or xgreek) (only in the
"stock" strings, "Περιεχόμενα", "Κεφάλαιο") and lualatex from texlive (not
with lualatex from the Arch Linux package).

Most strangely, if I do not use the titlesec package, the problem can be
seen only in "Κεφάλαιο" but not in "Περιεχόμενα" (!!)

It is not a matter of the font (or, at least, of the several I have tried).

`lualatex --credits` gives for stock texlive lualatex:
----------------------------
This is LuaHBTeX, Version 1.17.0 (TeX Live 2023)
Compiled with libharfbuzz 7.0.1; using 7.0.1
Compiled with libpng 1.6.39; using 1.6.39
Compiled with lua version 5.3.6
Compiled with mplib version 2.02
Compiled with zlib 1.2.13; using 1.2.13
Development id: 7581
----------------------------

while for Arch Linux lualatex:
----------------------------
This is LuaHBTeX, Version 1.17.0 (TeX Live 2023/Arch Linux)
Compiled with libharfbuzz 8.1.1; using 8.2.1
Compiled with libpng 1.6.40; using 1.6.40
Compiled with lua version 5.3.6
Compiled with mplib version 2.02
Compiled with zlib 1.3; using 1.3
Development id: 7581
----------------------------

The crucial difference is probably in harfbuzz. But how can this impact the
output in the ways described above?

Thanks in advance.
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