Interesting to see that invincibility skills end if you attack or use a skill. Interesting to know for tanks who get celestial shield etc while raiding etc.
There's a debuff which has been in place for quite a while which prevents chain casting of celestial shield. Prevents any invincibility skills from being cast on the original target for a short time. Too long to be classed as chain-casting.
Prior to that patch they chained it. Last patch made it to once per raid, and this pretty much made the skill unusable on the tank in a raid, which is the original intent.
Totally invalidates servitor barrier as well. While you can still save lives with invul spells, servitors are not half as smart as real people. Even if you do tell them to stop attacking so they don't throw off their invul, they most likely will not. At least, not until after they get their one last hit to invalidate the spell.