Ant var access in groovy when running in parallel tasks

I'm trying to find a way to do use the below in my Ant build scripts, which means using groovy to modify the variable - this operation works fine when doing in serial, but when I use the parallel features it only ever sees the last value of the variable. This wasn't a problem when using javascript in the <script> tags and I'm trying to work out why groovy is treating it differently.

<for list="${file.list}" delimiter="${line.separator}" param="single.val" keepgoing="true" parallel="true" threadCount="3">
    <sequential>
        <test.single.target test.single="@{single.val}"/>

<macrodef name="test.single.target">
    <attribute name="test.single" default="false">
    <sequential>
        <var name="testVar" value="@{test.single}" />

        <groovy>
            testVar = project.properties."testVar"
            println testVar

            if (testVar.endsWith(".class")) {
                testVar = testVar.substring(0, testVar.lastIndexOf(".class"))
            }
            
            testVar = testVar.replaceAll("//","/").replaceAll('/','.')
            
            properties["testVar"] = testVar
        </groovy>

        <echo>$test.single</echo>

so when executed with

path/to/use/one/a.class
path/to/use/two/b.class
path/to/use/three/c.class

it results in:

path.to.use.three.c
path.to.use.three.c
path.to.use.three.c

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.