Plugin excludes does not always work for transitive dependencies

Excluding dependency A from a plugin works fine if the plugin has a direct dependency to A:

runtime(':mail:1.0') {
    excludes 'mail'
}

'javax.mail:mail:1.4.3' is successfully excluded.

But if A has a dependency to B, it does not always work to exclude B:

runtime(':mail:1.0') {
    excludes 'activation'
}

Plugin ':mail:1.0' has a compile dependency to 'javax.mail:mail:1.4.3', which have a dependency to 'javax.activation:activation:1.1'
Here I would expect 'javax.activation:activation:1.1' to be excluded, but it's not.

When changing plugin scope til 'compile', it works though:

compile(':mail:1.0') {
    excludes 'activation'
}

Looks like transitive dependencies are only excluded if scope matches plugins scope, but for direct dependencies scope doesn't matter.

User list thread: http://grails.1312388.n4.nabble.com/Exclude-jars-from-plugin-tt4630055.html

To reproduce:

$ grails create-app excludetest
Add the following to BuildConfig.groovy's plugins:
		runtime(':mail:1.0') {
		    excludes 'activation'
		}
$ grails war
$ jar tf target/excludetest-0.1.war |grep activation
WEB-INF/lib/activation-1.1.jar