Losing Active defesne buff
Solution:
Disable autocast of it before you zone. If autocast powers refresh during loading they will fire but without effect but still get cooldown. (applies to more than just active defence, hasten for example)
Actually, exiting a MA mission removes all temporary buffs from the player. I'm not entirely sure why, but probably to prevent some sort of abuse with buffing custom allies.
I noticed this due to losing Practiced Brawler and Elude whenever I exit a mission, though the latter is almost handy because it also prevents me wasting time on the crash.
I lose active defense buff almost every time i enter a MA mission. it's quite annoying having to wait for it to cycle before i can actually start the mission.