Your device is automatically using the SD Card as internal storage, this makes the mount directory change from
storage/sdcard0, this is how Android utilises the SD Card as Adoptable Internal Storage...
You will still have the space you want,
If you would like to continue using internal storage as your primary storage location, you should select Phone Storage.. This will use a Fake SD Card emulated on User Data, Mounting the Fake SD Card @
storage/sdcard0, however if you want to use the real SD Card as internal storage, it will mount the Fake SD Card @
storage/sdcard1 and the Real SD Card @
The main directory that changes is the
sdcard directory, If you send a file to
/sdcard/ it will write to the External Storage selection, either Real or Fake SD Card, depending on your selection.
I'm not sure how you are planning to use the card, but I imagine you may have thought the OS merged the available storage into a single larger storage device.. this isn't the case.