GuiceOneServerPerSuite ignores play.application.loader configuration

I am using Play 2.6.x with Scala 2.12, Scalatest 3.0.5, Gradle 6.1.

My play application uses a custom Application Loader as follows:

class DummyApplicationLoader extends GuiceApplicationLoader

This loader is configured in application.conf as follows:

In DEV and PROD modes, run from gradle or from the command-line, this works perfectly.

However when running as a Unit Test, this DummyApplicationLoader is ignored.

I have also tried overriding fakeApplication, and force this configuration to be added, and then log out the whole configuration, I confirm that the configuration is present.

However it simply seems to be ignored. I have pored through the Play Documentation but this behaviour is not explained anywhere.

Even if I, in fakeApplication, set the Builder to use Mode.PROD, the ApplicationLoader config is still ignored.

Relevant snippets of my Unit Test:

class LocationAPISpec extends PlaySpec with ScalaFutures with GuiceOneServerPerSuite

   override def fakeApplication(): Application =
      new GuiceApplicationBuilder()
        .configure("play.application.loader" -> "")