Not saying that video was an occurrence of it happening since it is hard to see for that particular video, but maybe you are new to Tide games because its literally a feature to sometimes spawn an enemy behind players when they are offscreen for every players camera. This has been confirmed by many people in vermintide 2, I myself have seen it happen to a teammate. They dropped down into the next room and I was still up on the ledge watching and I saw an enemy spawn in and instantly swing at them.
It doesnt matter at all if you believe it or not, that doesnt change the fact that it happens and they should look into removing it because it can be really cheap sometimes, which is what this post is about.
Edit: before you even try to say that this isnt a thing, here is an absolute mountain of evidence that says otherwise
Mauler popping in on the left side
Marauder spawns behind teammate infront of them
beastmen group spawning near tree
Rat ogre spawn
ratling spawn in the brief moment everyone looked away
Mauler spawn left side on the edge of the screen
Marauder spawning behind cart
Enemies spawning complaint
beastmen spawning
Stormvermin patrol spawning
Clan rats spawning
Beastmen banner group spawning
Packrat spawning
Here is a quick one I found that proves the spawn system in darktide works exactly the same. Its how the game spawns things. There is no arguing this
Darktide spawns enemies the moment they are offscreen