Yes this happened to me too. Endrega charged at me, I dodged to the side, and the Endrega disapeared after passing me by, saw it with me own eyes!

I think alot of the glitchy behaviour to do with monster spawning/despawning is to do with the fact that they are leashed to certain areas. Its just badly implemented,hopefully a later patch will fix it up a bit.

For instance,ive noticed more then once in the woods outside flotsam..you walk into a certain area and get mobbed by Endregas and run away a bit to cast a Yrden or lay some traps down,they will follow to a certain extent then turn around and become unresponsive even if you attack them,eventually dissappearing or dieing easily.

Or sometimes an Endrega who has been lead away from its area a tiny bit will charge at you,and if you dodge it and turn around to attack it just dissappears into thin air due to it being unleashed from its area etc

Same goes for Nekker nests in Flotsam forest..which are buggy as hell imo. They spawn when you get on top of their nest,then if you roll away a tiny bit some bug out and become unresponsive,some attack,and others re-bury themselves.

Also on top of that,ive noticed that if you quick save right before a nekker nest fight..get killed in said nekker nest fight,then reload that save..none will spawn over that nest at all. However,if you run off to do something else..later you might find yourself being swarmed by a bunch of angry nekkers that shouldnt be in the area you are exploring lol.

All in all,i think all the bugs are due to the leashing system CDPR have put in place for certain monsters,especially in Flotsam Forest. Guess its because they dont want us kiting an army of angry monsters around flotsam forest and crashing the game lol.

Its a bug with the Endrega. I have many disappeared right in front of my eyes as I was attacking them. Also the "perimeter" where these monster chases you is implemented is pretty crappy. If you know where that line is, you can cheese all them to death by crossing back and forth the line. I hope they fix both the bug and make this "cheesing" tactic standing at the magical border not so obvious. It takes away the realism of being chased and battles.

Just wanted to say that the... problem (or is it intended?) still exists. Endregas still vanish into thin air right in front of the player.

I'm just here to confirm that I've seen this behaviour, as well. All the monsters you encounter are, as others have already explained, limited to a certain area - if the player ventures outside that area, they are scripted to retreat and despawn, as it were. Nekkers usually burrow underground, harpies fly off into the air, and the endregas seem to be a bit bugged, because they start running away only to suddenly vanish before your eyes. It's a bit annoying, and should be attended to, sure, but it generally doesn't bother me; being aware of this, I just make sure not to start 'running away' from the monsters if I intend to fight them, since I know I run the risk of triggering their despawn script if I do.

Yeah I've had this happen to me also. I was worried it might be caused from a corrupted save file or anything. I don't need to start over again do I?