can zone bosses disappear over time?

[dragonus]

I don't know if it is the rule: if you somehow got out of the boss level to the world map before you kill the zone boss (or before he/she appeared in your LoS), and return later, you will finish with the zone, but the boss will not be there any more. In my recent Doombringer run, first I missed The Shade. When I started to receive magic shots from invisible source in that boss area in the 3rd floor of Kor'Pul, I realized I didn't have a pickaxe or even a regen infusion on me. So I tp out to the worldmap and grab the needed from Derth, went back, I couldn't find The Shade anywhere in Kor'Pul. Then later, I did something stupid again in the second floor of Mark of the Spellblaze. I think I was just about two Rush away from the normal standing location of Grand Corruptor (not yet in my LoS though), I touched a Tentacle statue and got killed in the wormhole. While in the Eidolon Plane (I was playing Adventure mode), I chose to return to worldmap. When I went back to the second floor of Mark of the Spellblaze, Grand Corruptor was nowhere to be found. This hasn't happened to me before, I just wonder if it is the case for optional zones (both of them are optional for Doombringer)?