I just did a factory reset from within Fairphone Open OS thinking it would reset my phone to a somewhat near initial state … wrong, OS and TWRP are the same and encryption is still there, only every other user-related stuff is gone and the OS starts with the initial setup.
Dear diary, “factory reset” nowadays seems to be a funny thing not 100% related to the term I once knew, I’m feeling old 
…
So I wiped everything in TWRP and formatted data again to get rid of the encryption.
This time I installed the newer LineageOS build from over at the OTA thread like @sky suggested, and indeed, FM Radio and encryption do work.
Awesome 
!