If you refer to a WB, yes, they do that, it's like a "recycle" of some sort, no Idea why but they do. The good news is that if that WB is "bugged", this "recycle" makes it un-bugged. That's how I got the achievement on a bug one.
I got it today in the slavers camp in Shadowfen. Banished while I was mid-air charging with my 2hander (arghhhhh... what???).
To mention, the respawn timer was MUCH shorter than an after-kill boss respawn, so I guess its more than worth the trouble to avoid world bosses bugging with the lack of recycle.