There's a chance you have desynced from the server and this has a chance of resyncing. Record and report these players if they use this exploit to kill other players. If you notice a player's blip inside a locked out and they're not on the roof, they could be shooting from inside the walls. Go to Single Player then back to Online to clear the Recent Players list. If you see any enemies, they will be able to join you from the same Recent Players list. Glitches and Exploits Awareness Players > R1 (x2).
Even though changing Third Person Controls and other Settings is suggested, these tips are written as if default controls are used. If you see misinformation please leave a comment, directly PM me, or my Discord is SmurfInn#4360.