I actually have a similar glitch, but rather than Jacob Zenger, I am stuck at 90% with Clipper Wilkinson even though he is a level 11 Assassin. I really hope the patch fixes this because Ii'm at 99% complete and would like to have the game platinum'd this week.
"You may take my life, but I will take your soul!"-Paptimus Scirocco, Zeta Gundam
After seeing the patch breakdown http://forums.ubi.com/showthread.php/726389-Assassin-s-Creed-III-Thanksgiving-Patch-Spoilers I wanna be hopeful, but I just don't see it listed as one of the glitches to be fixed... *sigh* I'm in the same boat as you, mate, I'm at 99% and Jacob's the only thing getting in between me and my plat!