Italic text is smaller than regular text when using clearpage and the scrlayer-scrpage package

Question

I am using Luatex and KOMA-script. I have the issue that the italic font is consistently set to size 10pt it seems, and I don't understand it. In my M(not)WE below, the issue is present, but it isn't when removing the line that loads the scrlayer-scrpage package or removing the \clearpage. I'm not using the package in this example to keep it simple, but in my main document I want to use it. What is going on here? Also, this issue doesn't happen for most other fonts.

\documentclass[a4paper, 12pt]{scrreprt}

%%%%%%%%%%%%%%% load packages %%%%%%%%%%%%%%%
\usepackage{fontspec}
\setmainfont{Latin Modern Roman}

%%%%% Head and Footnotes %%%%%
\usepackage[]{scrlayer-scrpage}

%%%%%%%%%%%%%%% begin of document %%%%%%%%%%%%%%%
\begin{document}
Some Text
\clearpage  % or cleardoublepage or newpage, all produce the problem
This is a text in normalsize. \emph{This appears smaller than it should. }\textit{This appears too small as well.}\\
Look at the size of Letters: u\emph{u}\textit{u}

\end{document}

Answer 1

Fonts with optical sizes are sometimes a bit complicated. Declare the slanted font explicitly:

\documentclass[a4paper, 12pt]{scrreprt}

%%%%%%%%%%%%%%% load packages %%%%%%%%%%%%%%%
\usepackage{fontspec}
\setmainfont{Latin Modern Roman}[SlantedFont=LatinModernRoman/I]

%%%%% Head and Footnotes %%%%%
\usepackage[]{scrlayer-scrpage}

%%%%%%%%%%%%%%% begin of document %%%%%%%%%%%%%%%
\begin{document}
Some Text
\clearpage  % or cleardoublepage or newpage, all produce the problem
This is a text in normalsize. \emph{This appears smaller than it should. }\textit{This appears too small as well.}\\
Look at the size of Letters: u\emph{u}\textit{u}

\end{document}