After some chapter headings document leaves page empty before text

Question

First of all, my problem is not that at the end of a chapter, i.e. before the chapter heading, a page is empty (e.g. problems like https://stackoverflow.com/questions/1358694/latex-remove-blank-page-after-a-part-or-chapter) caused by the 'twoside' option. I've tested my problem with the 'oneside' option and it still persists.

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So my actual problem is that in my document (a custom template I am required to use, based on the KOMA-Script class scrbook. I've checked the class file but couldn't find anything.) at some point, the remainder of the page is empty after a chapter heading, and the text only starts at the next (even numbered) page. The peculiar thing is that this behaviour is not consistent over all chapters, but rather starts after chapter 3. I've tested my code with only 'lipsum' segments to ensure that no odd figure or table is the culprit, but this didn't solve the problem.

\documentclass[
  a4paper,            % paper size (a4paper, a5paper)
  english,            % define the document language (english, german)
  twoside,            % use onesided or twosided layout (oneside, twoside)
]{tumbook}            % document class I have to use
\begin{document}

\frontmatter
\maketitle
\chapter{Abstract}
\input{chapters/abstract.tex}
\tableofcontents

\mainmatter
\chapter{Introduction}
\input{chapters/intro.tex}

\chapter{Theory}
\input{chapters/theory.tex}

\chapter{Data Sample}
\input{chapters/datasample.tex}

\chapter{Methods}                       % The problem starts here
\include{chapters/methods.tex}

\chapter{Results}
\include{chapters/results.tex}

\appendix
\chapter{Appendix}
\include{chapters/appendix.tex}

\backmatter

\printbibliography

\end{document}

The individual files included were all tested with dummy content of the form

\lipsum[1]

\section{A section}
\lipsum[2]

\section{Another section}
\lipsum[3]

I don't expect anyone to have the finished solution since the problem most likely lies with the custom document class, but I was wondering if someone could give me some pointers as to where the problem might come from.

Answer 1

For the first three chapters you \input the text. For the remainders you \include the text. \include always starts a new page. Just use \input all the time.