In a defaults file I have e.g:

variables:
  header-includes:
    - \newcommand{\mydef}{My definition}

This will provide for the TeX document the command \mydef.

In my Markdown document I have

---
header-includes:
  - \newcommand{\yourprop}{Your proposition}

...

This will *not* provide  a command `\yourprop`, instead every time when in the source `\yourprop` appear it will be replaced *by pandoc* with „Your proposition“.

Where on earth is this behaviour coming from?