I now understand the underlying reasons why the available storage is cut in half, but I still wished it was otherwise. And it's ultimately the main reason why I sold my 6.1 again.
found this elsewhere
It is by design, but not by Nokias!
AndroidOne uses FBE (File Based Encryption) which is at the moment not compatible with Adoptable Storage (i.e. format as internal)
Its documented here :
Android 7.0 and later supports file-based encryption (FBE). File-based encryption allows different files to be encrypted with different keys that can be unlocked independently.
This article describes how to enable file-based encryption on new devices and how system applications can be updated to take full advantage of the new Direct Boot APIs and offer users the best, most secure experience possible.
Caution: On devices running Android 7.0-8.1, file-based encryption can't be used together with adoptable storage. On devices using FBE, new storage media (such as an SD card) must be used as traditional storage.
Devices running Android 9 and higher can use adoptable storage and FBE.