Yea, it is quite simple actually. (And a by product of how level sync is programmed) When the syncée delevels, the level sync is re-applied and at this moment all buffs are cleared. It is however funny that the game can handle leveling up, because all spells you could cast at level X, are available at level X+1. While it takes the safe bet and removes all buffs if you go X to X-1 just in case X got a spell X-1 doesn't.

SE could update it to not remove buffs if your level change is + or - 1 I guess. Would solve this problem, but allow people to slightly cheat by casting level 14 spells and syncing to 13 and having a slight edge over the monsters. (But I can't see how this is really a problem)