When a macro with an optional argument is replaced, if it is followed by a \n the whitespace is discarded.

To see the issue feed

\newcommand{\macro}[1]{bla #1 bla}
\newcommand{\macroopt}[1][]{bla #1 bla}
A \macro{B}
C

A \macroopt[B]
C

A \macroopt[B] C

to

pandoc -f latex -t plain

to obtain

A bla B bla C

A bla B blaC

A bla B bla C

Note that, following the macro expansion rules of LaTeX,
there should be a space between bla and C in the second line as well.

Tested with

pandoc 1.12.3.3
Compiled with texmath 0.6.6, highlighting-kate 0.5.6.1.

Great thanks a lot!