It is indeed a Great Sword, also says so when you summon it and look at it's description. So should work fine with Devoted, and also with the modal. I just did a quick check and made myself a druid / devoted hireling with Great Sword proficiency, which got correct accuracy and use of modal for both spell and scroll Firebrand. And in the past I've had a main character with Great Sword proficiency who occasionally used scroll Firebrand; not devoted, but definitely used modal for that too.
So if it's a bug, it's not an entirely general one. Might be an issue specifically with your game files perhaps, I'd try verifying the game files for a start (if you're using Steam, that is).
Edited by Loren Tyr, 13 July 2018 - 03:56 PM.