The legendary beast The Mire Terror is missing

I died 3 times to them. I changed my stances and tried to go back and they weren’t there. Tried restarting, dying in the area etc

Took the elevator up from Viscid Bog to Gorge Crash site and this happened to me. On PS5 as well. I double-backed to where he should've been and I was still able to get the stim upgrade. Even got the XP for "defeating" him. Definitely could've been worse with how this all turned out, but I wish I was able to earn those rewards the right way.

I'm having a similar issue. I got to where you can either head to fight the Mire Terror or go up the elevator, so I went to go fight him and I couldn't get there. When trying to use my force ability to raise those spires to get there, about 75% of the time it just simply wouldn't work. It would highlight them in blue and give the "R1 + triangle" prompt, but most of the time Cal would simply do nothing or it would play the animation of when you try to use a force ability but it doesn't work. It would randomly decide to work, but I could never string enough together to get across all 3 spires and to the Mire Terror. Then I decided to head up the elevator to see if resting than coming back would help and as I headed towards the Gorge Crash Site meditation point, it popped up saying I'd finished that Rumor and I got XP for it, and The Mire Terror shows up in my tactical guide even though I never saw him

EDIT: Adding to this, I did figure out the solution to one of my issues. When I was trying the force ability to lift the rocks and it wasn't working, I had Blaster and Dual-Wield equipped for stances, I tried switching stances to see if that changed anything, and amazingly, it did. I switched to Crossguard and Double Bladed and then everything worked fine and I was able to finally get to The Mire Terror area. However, as mentioned by others, he was nowhere to be seen, since according to the game I had already "beat" him. I was able to get the extra stim and leave without any battle

Similar and as others have said, I took the elevator and it seems the boss decided to just walk to the swamp and auto eliminate itself as I was taking the elevator out of there. I still got the stimulus pack but I don't recall getting "Mire Terror Defeated" notification. This is important as the game keeps track of it to get a trophy. PS5 console.

Same on PC came from lukerhulk. Found, fought and lost. Went up George crash site and came back down they were gone.

Same on x-box series s for me.