Weird 'wall hack' like bug

[Zakificus 10]

Unfortunately I disconnected before I could write down server name. But I was up to date with beta and mod versions. When I connected all zombies and animals had white spheres around them. They were the faded transparent ones like you see at the edges of the screen indicating zeds nearby. It was visible through hills/walls and varied in how clearly it was visible and from what range, though usually about 100 meters I would estimate.

Also notable I found a bike that had a similar red sphere.

It didn't reoccur on any server since. I didn't see any players so I can't confirm If it affected all entities or only those automatically spawned.

Also notable, it was pretty laggy during the connecting/loading phase so that may have had some bearing on what caused it.