So I just had this happen twice to me. The second time was after finding this short thread, and I was able to realize it happened before wasting too much time on the fight.
Basically, if you get seen before the cutscene triggers, the fight happens, then the pre-fight cutscene after it is over. I can confirm the cutscene from after the first completion of the fight is the same as the one before my third go just now starting. I'm not sure what saw me the first time, but the second time, an abomination climbed up on the ledge where the characters are during the cutscene, so even though I hadn't gotten there yet, the fight began. The first time, I was on the ledge, inside the "tower" for lack of a better term, when seen, but the fight triggered before the scene.
During the second fight, I noted that the quest journal still told me to "Approach the rift" rather than "Defeat the Tshar." So literally, if you get spotted, by anything, before reaching the point where the cutscene is meant to trigger, you get stuck after the fight.
On the third attempt, I was cautious and rather than going on the higher route, I took the left side path, and was able to trigger the scene first. Though it also seemed like there were no wanderers close by this time around, so it may just be bad timing that they are close enough to spot the characters.