Up to Lollipop, native `app2sd` only moved `.apk` files as encrypted `.asec` files to `/mnt/sdcard/.android_secure` folder. On every boot these encrypted file were decrypted and placed in a temporary filesystem mounted at `/mnt/asec/` to which the symlinks from app directories in `/data/app/` were pointing.

Starting with Marshmallow, the native method of moving apps to external SD card works only if SD card is formatted as `Adoptable Storage` <sup>([1][1])</sup>:
> new installs into ASEC containers are no longer supported; use adoptable storage instead.

[Adoptable Storage][3] is a Full Disk Encrypted SD card, completely managed by `vold`, not visible to user. See [here][2] how it differs from traditional [Portable Storage][4]. You can format SD card as partially Adoptable and partially Portable too.

Like the pre-Marshmallow `app2sd` phenomenon <sup>([2][5])</sup>, apps only with `installLocation` set to `preferExternal` or `auto` in app's `manifest` are moveable:
> Apps can be placed on adopted storage media only when the developer has indicated support through the `android:installLocation` attribute.

If you want to overcome this obstacle, use a third party app like `Apps2SD` or `Link2SD`. Or an even better approach is to move `/data/media` instead of apps. Create an extra partition on external SD card and `on post-fs-data` mount it to `/data/media` using an `init.d` script or Android's `init` service. Other bigger directories `/data/app` or `/data/data` can also be moved in the same way.

**PS:**  
If you want to keep files encrypted on external SD card, this can be achieved through native methods `dm-crypt` (Android's `FDE`) for block device encryption, `ecryptfs` for stacked filesystem encryption, or third party solution like `EncFS` for stacked encryption. New native method of filesystem encryption `fscrypt` (Android's `FBE`) may also work, but I haven't tried.


  [1]: https://android.googlesource.com/platform/frameworks/base/+/refs/tags/android-9.0.0_r42/core/java/android/content/pm/PackageManager.java#732
  [2]: https://fossbytes.com/android-sd-card-internal-storage-adoptable-storage/
  [3]: https://source.android.com/devices/storage/adoptable
  [4]: https://source.android.com/devices/storage/traditional.html
  [5]: https://developer.android.com/guide/topics/data/install-location