I recently wiped my FP2 (formatting Data, System and Cache) and installed LineageOS 16. Installation was successful, but I subsequently was unable to encrypt the phone. When trying to start the encryption, it would just “soft crash” and not perform the usual full reboot and encryption procedure.
I found the culprit after a lot of trial and error: Following the official guide, I had flashed the latest recovery file: TWRP 3.3.1-0. Formatting the Data partition with this version seems to modify it in a way that makes it impossible for LineageOS 16 to encrypt it. After flashing TWRP 3.2.3-0 and formatting Data again the encryption procedure worked perfectly fine. I tried this several times with both TWRP versions and it’s 100% reproducible.
(I’m aware that all TWRP versions are unable to decrypt the Android 8 / 9 encryption. This is unrelated.)