Barbies World

Steps to reproduce are unclear.
Sometimes there are monsters displayed not in their actual location, including radar.
Shooting them leads to nothing.
But, if you shoot somewhere else (like empty corners), and it matches to actual location, they die normally.
Also they can see (and shoot at) you from real location but you can't see them This issue happens rarely, on some specific maps, and when said monster walks or teleports to specific locations they seem to glitch out

I guess that's a replication issue: your local UT tries to predict movements of players, NPCs, projectiles and other movable objects, but that is in case of differences adjusted and corrected by the server periodically. Each UT client/server connection has 1024 channels to communicate, and if there is a heavy load because of a lot of things to update some may be left out and the local predicted location is different to that what the server "thinks".

What is your setting of "ConfiguredInternetSpeed" in UnrealTournament.ini?

It's 20000.

And – most importantly – other players on server also see monsters in exact same spots during this issue.