Hey @phonefrog,
how did you encrypt the phone?
Do you use any additional apps to split the de-cryption password from your screen-unlock password?
I just remember that I hat some issues with Snooperstopper. As far as I understand on LOS this isn’t a problem.
Up to now I did not encrypt my phone again (here I described my last update to FP Open 17.11.2) as I wasted so much time with fighting with these kind of issues. (additionally I did not yet flash the latest TWRP version -> 3.2.0-0)
On the other hand I never had a real problem with de-crypt when starting TWRP. As mentioned by the others it always ask for a PIN when bootinh into TWRP.
If you will not find a solution until the 19th we can check it together at the FP Hamburg Meetup. Another guy (I know him from a XMPP chat room) will probably join, who has an encrytped FP2 with FP Open and without gapps. He currently uses the same password for screen unlock and de-cryption on boot.