When you cast shield with GSF Abjur, it gives you +8 AC. Due to I think +4 of it being armor ac and how it applies it, the +4 lingers for an extra turn or so.The problem there is that when you re-cast shield, it doesn't refresh that +4 AC or overwrite it. Which means that when that turn expires you lose that 4 ac, and only have +4 AC overall from shield.
If the wacky duration could be fixed, or just have it refresh on cast, that would be fantastic for any kind of mage that uses extended shield!