I think you're right. I'm currently listening to the Audiobooks for the first time (and can highly recommend them), and I'll check back when I get to the end of First Lord's Fury, but I don't recall Sextus's letter to Tavi being described, only that Ehren somehow got himself new orders about what to do with Acquitaine, and I can't remember if they were coded in Sextus's letter to Attis, his letter to Tavi, or a separate message altogether.
Overall, I'm not sure it's too important. Tavi didn't need much from Sextus by the time he became First Lord. Personally, I think every bit of the message to Tavi was actually sent by Alera, and Ehren's message might've been a phony.