“Formatting” is not the same as “encrypting” - it seems that Android 9 on Fairphone 3 does not have the possibility to encrypt sdcards as portable storage. According to this article, all modern Android phones will encrypt the data on the internal sdcard by default. The article also gives some hints no how to get the contents of the card encrypted using third party applications.
Not sure if this is true however for the Fairphone 3. Somehow on the Fairphone 3, the “adoptable storage” mechanism seems to be not standard. Reading on in the official Android documentation on adoptable storage. Perhaps we need an official reply from Fairphone 3 support on how the system deals with SDcards?
Found some more threads dealing with the same question: