Why does my formatting break when I change compiler from pdfLaTeX to LuaLaTeX?

Question

I'm writing a document in Overleaf using a document template I've been given, and I need to use the formatting style set out in that template. I've had to change the compiler I'm using from pdfLaTeX to LuaLaTeX so that I can make use of certain linguistic Unicode characters.

The problem I'm having is that changing the compiler seems to have broken the formatting of the template, and I'm not sure how I can fix it.

The only change I made in the preamble to facilitate the new compiler was that I replaced this:

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}

With this:

\usepackage{fontspec}
\newfontfamily\abr{Noto Serif}
\newfontfamily\og{Noto Sans Ogham}

Now my title page has changed, the title is no longer bold or using the correct margin sizes. Chapter titles in the table of contents are all lowercase, and section titles in the main body are lowercase too. Aside from these specific issues, the primary font of the whole document has also clearly changed. These are just the issues I've noticed, I suspect there are more subtle changes I haven't noticed yet.

How can I get it to look like it did with the old compiler?

Here are some pictures for reference.

Before (correct formatting):

After (formatting broken):

EDIT: Adding minimal code to enable recreation of the issue.

The issue should be reproducible just switching between compilers without making any changes to the code below.

\documentclass[a4paper,headinclude,footinclude]{book}

% document packages



\usepackage{iftex}

\iftutex
 \usepackage{fontspec}
 \newfontfamily\abr{Noto Serif}
 \newfontfamily\og{Noto Sans Ogham}
\else
 \usepackage[T1]{fontenc}
 \usepackage[utf8]{inputenc}

\fi

\usepackage[english]{babel}
\usepackage[hyphens]{url}
\usepackage{setspace}
\usepackage{graphicx}
\graphicspath{{./chapters/images/}}
\usepackage{titling}
\usepackage[toc,page]{appendix}
\usepackage{caption}
\usepackage{subcaption}
\usepackage[
  eulerchapternumbers,
  eulermath,
  subfig,
  beramono,
  pdfspacing,
  floatperchapter,
]{classicthesis}
\usepackage{arsclassica}
\usepackage[  % check the README file about margins
  left=3cm,
  right=3cm,
  top=3cm,
  bottom=3cm,
  heightrounded,
]{geometry}
\usepackage{lipsum}  % for dummy text, you can remove this

% macro definitions
\newcommand{\figurePath}[1]{figures/#1}

% use one and a half line spacing
\onehalfspacing

% set your title, author and publication date here
\title{Development of Natural Looking Titles and Reports about Nothing; with an Application for Deterring Authorship in Other Word Processors}
\author{Andrew Daly}
\date{\today}

% start the document with the title page
\begin{document}
\begin{titlepage}
  \centering
  % main logo
  \vspace*{3em}
  % main title, author and date
  {\color{darkgray} \Large \bfseries\selectfont Team Report} \\ [5em]
  {\sffamily \bfseries \selectfont \huge \singlespacing \thetitle \par}
  \vspace*{2em}
  {\Large \theauthor} \\ [4em]
  {\textbf \thedate} \\ [4em]
  % teams and members
  \parbox{0.25\textwidth}{
    \centering \color{darkgray}
    \textbf{Team A} \\ Greg Thompson
  }
  \parbox{0.4\textwidth}{
    \centering \color{darkgray}
    \textbf{Team B} \\ John McAvoy \\ Cliodhna Dempsy
  }
  \parbox{0.25\textwidth}{
    \centering \color{darkgray}
    \textbf{Team C} \\ Paul Buttler
  } \\
  \vfill
  % secondary logo and footer

  {\color{darkgray}
  Centre for Mocked Up Team Reviews \\
  College of Teamwork, International University of Jupiter
  }
\end{titlepage}

% the front matter of the document
\frontmatter
\chapter*{Abstract}
This is an abstract, there should be about a paragraph of text here. 
\lipsum[10]
\tableofcontents
\chapter*{Acknowledgements}
Thanks folks!

% these are the included chapters
\mainmatter
\chapter{Review of the Literature}\label{ch:litrev}
\section{All About the Dictionary}
This is some text about a dictionary. \lipsum[5]
\section{Teamwork in the Context of Dictionaries}
It would be difficult to write a dictionary on your own. \lipsum[10]
\section{Another Title I Came Up With}
I'm running out of things to say here. \lipsum[9]
\lipsum[6-8]

\appendices
\chapter{Placeholders}
\section{Unused References}
\begin{enumerate}
    \item List item 1
    \item List item 2
    \item List item 3
\end{enumerate}

\end{document}

Answer 1

the classicthesis and arsclassica packages are not really set up for Unicode fonts, and LaTeX does warn you that sans serif is not working,

LaTeX Font Warning: Font shape `TU/iwona/m/n' undefined
(Font)              using `TU/lmr/m/n' instead on input line 65.


(/usr/local/texlive/2021/texmf-dist/tex/latex/microtype/mt-LatinModernRoman.cfg
)

LaTeX Font Warning: Font shape `TU/iwona/b/n' undefined
(Font)              using `TU/iwona/m/n' instead on input line 65.

That is the first one saying it can't find iwona so will use latin modern, then the second one saying it can't find bold.

However you can use fontspec to set the sans serif to the OpenType version of iwona then it does set up bold as well

Output from lualatex:

\documentclass[a4paper,headinclude,footinclude]{book}

% document packages



\usepackage{iftex}


\usepackage[english]{babel}
\usepackage[hyphens]{url}
\usepackage{setspace}
\usepackage{graphicx}
\graphicspath{{./chapters/images/}}
\usepackage{titling}
\usepackage[toc,page]{appendix}
\usepackage{caption}
\usepackage{subcaption}
\usepackage[
  eulerchapternumbers,
  eulermath,
  subfig,
  beramono,
  pdfspacing,
  floatperchapter,
]{classicthesis}
\usepackage{arsclassica}


\iftutex
 \usepackage{fontspec}
 \newfontfamily\abr{Noto Serif}
 \newfontfamily\og{Noto Sans Ogham}
 \setsansfont{iwona}
\else
 \usepackage[T1]{fontenc}
 \usepackage[utf8]{inputenc}

\fi

---

that fixes the main issue, although arsclassica still has some low level code such as

%************************************************************
% Chapter numbers
%************************************************************
\let\chapterNumber\undefined
\ifct@eulerchapternumbers
\newfont{\chapterNumber}{eurb10 scaled 5000}%
\else
\newfont{\chapterNumber}{pplr9d scaled 5000}%
\fi

which is using internal names and scaling commands to force legacy 8bit fonts, you probably should use the \iftutex block in the preamble above to use fontspec defined fonts for \chapterNumber as well.