Fiio itself has never confirmed that 256GB cards work in the firmware updates as far as I know. Even in their support forum [1] they only state that 200GB cards are compatible. This is also confirmed by a user there. Personally, I don't see why it shouldn't work. Unfortunately, I can't test it myself because I don't have an SD card, so no guarantee.
Side note: Fiio's library system is known to be quite slow - even with the new models. With such large cards, updating will take forever. In the Fiio forum, a user reports 25 to 30 minutes for 150GB. So if you want to manage a larger music collection, I would look for other DAPs that can manage it better.
[1]
http://fiio.me/forum.php