Thanks for letting us know about the issue you are seeing.
What model is your Roku Express+? Screen mirroring is only supported on model 3910 and not on the model 3710. However, screen mirroring is only supported via HDMI output and not on the A/V output.
For more detailed information regarding screen mirroring you can visit the support page here: https://support.roku.com/article/208754928-how-do-i-use-screen-mirroring-with-my-android-or-windows-...