Ah-ha!

The issue was in the use of the var :

<var name="testVar" value="@{test.single}" />

, I've replaced it out with the following:

<local name="test.single"/>
<property name="test.single" value="@{test.single}"/>

And the values are getting get/set correctly, it seems there is an issue with how the var extension is treated in parallel runs.