I was soloing ToS Heroic when I encountered this bug.
I was attempting to solo the Desolate Host. I started in the Spirit Realm and downed Soul Queen Dejahana. The Host never came out at 30% and the encounter ended. No loot was passed out. The boss DID NOT reset. I even exited the instance and tried to reset the instance and the boss still did not return. The game didnt register as the boss being defeated, so the doorway to the Maiden of Vigiliance did not open.
Sorry for necro, but I wanted to add that this also just happened to me on this character.
10-man Normal. Porting out didn’t reset him, re-logging didn’t reset him.
Hey same. July 16th and still having this issue. RIP my mog run 
Sorry for the bump, but this just happened to me. Kind of sucks.
Same here, did it on Heroic and Mythic
It’s resetting for me constantly. I don’t know how to stop the reset.
It just reset for me. Had him almost down. Very, very frustrating.
For me worked using at least once the Spiritual font and stay in the same realm of the boss, just look for this https://wow.zamimg.com/uploads/screenshots/normal/635235.jpg
I tried this and was able to get the boss down successfully. Thanks for the tip!
Still happening. I went into the soul realm to fight Dejahna for funzies and didn’t realize how badly I screwed myself. 
RIP Guardian’s Seal/mog run.
Had the same happen just now. went straight into Spirit Realm and dropped Soul Queen Dejahana first. Back in the real world Engine of souls disappeared, no Desolate Host. The three adds were there.
If anyone is still having problems like I have been, I just managed to down this boss on 10m normal. I started in the spirit realm fighting the soul queen. I did so for about 15% of her health then headed back to fight the Enigma and never went back through a spirit font. At about 25% left on the bosses health the host spawned and I pulled him to the middle of the room where I was able to finish him off. Hope this helps and happy farming.
Fix this crap, Blizzard! It has been reported for almost a year. There goes a wasted week getting mogs in ToS!
Yup, got the same bug, playing with my Hunter. I logged out in the dungeon and in the next day the boss came back… The second try I hurt the Queen until she had about 30% of life and went to meet the desolate host, but it disappeared quickly. I’m not sure if my pets killed the other boss by mistake, it’s my first ride at this content and things were a bit hectic. I’ll try the suggestion above, but it’s upsetting this bug wasn’t fixed even after such a long time of the first post. That and the Emerald Nightmare soloing. 
March 21’ and this is STILL and issue. Beyond annoying. :\
Just happened to me also… very frustrating
Just tried to do the encounter still bugged.
Just happened to me
Had same issue today. Still bugged
Same here. Still bugged. Normal Raid run wasted.,
