If I don't remember wrong, if you evaluate this expresion:
<targetRef>_12_ContentInput</targetRef>
<assignment>
<from xsi:type="tFormalExpression">#{transportMap}</from>
<to xsi:type="tFormalExpression">_12_ContentInput</to>
</assignment>
It is converted to string.
You could try this one:
<sourceRef>transportMap</sourceRef>
<targetRef>_12_ContentInput</targetRef>
Demian