Android 6.0 Marshmallow adoptable storage - Why is it still using internal storage?
As mentioned in the comments of the question, this is by design.

According to the docs:


  Apps can be placed on adopted storage media only when the developer has indicated support through the android:installLocation attribute. New installs of supported apps are automatically placed on the storage device with the most free space, and users can move supported apps between storage devices in the Settings app.


In Nougat+, we could even force most app to be movable even without the developers support by toggling Force allow apps on external switch inside developer options.

See here for more: https://www.reddit.com/r/GalaxyS7/comments/5qi1mj/with_nougat_now_we_can_finally_transfer_90_of_the/