I have never experienced this before, not even after fights with multiple tanks and runners where the physics were pushing the games limits. I have had terrible luck with reapers in this world, but I spent roughly and hour and thirty minutes to two hours trying to kill this reaper. He was about to do his final thermobaric blast so I lit him up and killed him myself so I could get the better loot. This was a very tense moment for me because the previous reaper I had killed blew himself up and I got jackshit from it. I had successfully killed him, his falling animation and explosion had played. The exact moment its body had hit the ground, it despawned, it was not popping in and out of existence either, it just disappeared completely. I don’t expect any advice as the game has autosaved by now and im not going to reap the benifits of my labor, but I just want someone to understand my frustration.
TLDR, I grinded my ass off to kill a reaper, just for its body to disappear instantly after I killed it.