Interface with tex_math_single_backslash seems problematic

[aubertc]

Consider the following, that uses the tex_math_single_backslash:

master.md:

<!--
Compile with
pandoc master.md --filter pandoc-include -f markdown+tex_math_single_backslash -o out.pdf
-->

\(\lambda\), $\lambda$

!include included.md

included.md:

\(\lambda\), $\lambda$

Then the \(\lambda\) inside master.md is correctly rendered as math., but the one that is included gives:

Error producing PDF.
! Missing $ inserted.
<inserted text> 
                $
l.56 (\lambda