Previously there were two dependency managers being configured (one for regular, one for forked). A dependency manager is not needed in the forked JVM though as Grails doesn't do a second resolve.
This additional check for externally configured dependencies was added with this commit: 40c3c1e. The issue is that when forked, isDependenciesExternallyConfigured is true and therefore Ivy is used when Aether should be used for inline plugins. Removing this check fixes the issue and there did not appear to be any side effects of configuring a dependency manager even if dependencies were configured externally.