Having the same problem, at least I think. What happens for me, is that when I get inside a vehicle and leave, the other players still see me inside the vehicle, even if I'm outside, so I'm basically invisible + invincible. I can kill them, as you experienced too, and they can shoot through me. They can, however, kill the ghost-entity that is sitting inside the vehicle (only they can see it, not the affected player himself). If the affected player blows up the vehicle by himself, he won't take damage (probably the damage is calculated locally, and sent to the server like "I shot player XXX", which would explain why they can kill the ghost entity sitting inside the car, even though it doesn't really exist). It also appears this only happens to the host, and was 100% reproducable for me. Every time I got into a vehicle and got out it happened, but never to anybody else. This also often happened together with the bug where I couldn't leave the vehicle, and whenever I selected "Get out" i was instantly teleported back in, as long as there was a driver, though this bug didn't happen every time, so they might be completely unrelated bugs. Anyway, as it is now, the game is basically unplayable.

I observed similar problem in Arma 2, probably old bugs.

Negative, this bug is new. I have observed similar things in arma 2 and older versions of arma 3 too, but this time it is not random desync but a consistant bug. I cannot tell if it happens 100% of the times, but at least 90% of the times I got into a vehicle it happened to me, and also, it seems to only happen to the host.

I am now trying out running a dedicated server on my PC to solve this problem until BI fixes it. Also, CzechTroller, did this ever happen to you while you weren't hosting? did it happen to other players when THEY where hosting?

I can confirm this is a new bug. There is a problem with the client being able to send get out data with vehicles to the server (probably because they didn't change something important when undoing the gauranteed network messaging), usually for me anyway it happens after about 20 minutes. Was on a dedicated server, friend showed on my screen as sitting in a truck, while he told me he was over 600 meters away. I spawned another truck where he said he was just to see what would happen, sure enough when he jumped in that truck I spawned, his character warped over there and out of the old vehicle he was in.

DarkWiiPlayer, I cant get info what happens when friends. host, just because they dont have a good PC and they also have not so good internet and when hosting all players suffer from lagging, so only I host.

Hello,
was there UAV control taking part while in the vehicle? We are aware of a problem with UAV terminal being used while in vehicle.

Negative, at least in my case. While it is true that, at least twice I used a UAV from inside the vehicle, at least once we took a car just after spawning (with a pilot, no UAV operator) and the error still happened. I am not sure if I had used a UAV terminal before in that same mission from inside a vehicle, so maybe the problem just seemed fixed after respawning and manifested itself again as soon as I got into a new vehicle. But I have no idea how this error works, so I cannot say for sure.
Anyway, I am not currently able to run the game and test things because I have no access to my computer, but as soon as I can, I will try to test this out with some friends and see what I can find out.
I hope this is just the problem you are aware of and that you can fix it soon.

Some, could be related, info:

If I create a "getIn" EH for a truck for example on the client and send PV to the server with crew of this truck when I get in ("passenger" in this case, don't want to mess up locality), and then compare crew of this truck on the server, something interesting happens..

If the truck is local to the client, crew of this truck on the server is [] at the moment I enter the truck, i.e. server has no clue truck is occupied. Server gets update a little bit later on.

If the truck is local to the server (while EH is still client side) the server idea of crew inside the truck and client idea of it are synchronised.

Here is the code I used:

Client

truck = cursortarget; publicvariable "truck"; truck addeventhandler ["getIn", {inVeh = crew truck; publicVariable "inVeh"; 0 = 0 spawn {sleep 1; player action ["Eject", truck]; sleep 1; player action ["getInCargo", truck];}}];

Server

"inVeh" addPublicVariableEventHandler {diag_log [crew truck, _this select 1]}

if truck is local to the client the result is:

[[],[B Alpha 1-1:1 (KK) REMOTE]]
[[],[B Alpha 1-1:1 (KK) REMOTE]]
[[],[B Alpha 1-1:1 (KK) REMOTE]]
[[],[B Alpha 1-1:1 (KK) REMOTE]]
[[],[B Alpha 1-1:1 (KK) REMOTE]]
[[],[B Alpha 1-1:1 (KK) REMOTE]]
[[],[B Alpha 1-1:1 (KK) REMOTE]]
[[],[B Alpha 1-1:1 (KK) REMOTE]]
[[],[B Alpha 1-1:1 (KK) REMOTE]]
[[],[B Alpha 1-1:1 (KK) REMOTE]]

if truck is local to the server

[[B Alpha 1-1:1 (KK) REMOTE],[B Alpha 1-1:1 (KK) REMOTE]]
[[B Alpha 1-1:1 (KK) REMOTE],[B Alpha 1-1:1 (KK) REMOTE]]
[[B Alpha 1-1:1 (KK) REMOTE],[B Alpha 1-1:1 (KK) REMOTE]]
[[B Alpha 1-1:1 (KK) REMOTE],[B Alpha 1-1:1 (KK) REMOTE]]
[[B Alpha 1-1:1 (KK) REMOTE],[B Alpha 1-1:1 (KK) REMOTE]]
[[B Alpha 1-1:1 (KK) REMOTE],[B Alpha 1-1:1 (KK) REMOTE]]
[[B Alpha 1-1:1 (KK) REMOTE],[B Alpha 1-1:1 (KK) REMOTE]]
[[B Alpha 1-1:1 (KK) REMOTE],[B Alpha 1-1:1 (KK) REMOTE]]
[[B Alpha 1-1:1 (KK) REMOTE],[B Alpha 1-1:1 (KK) REMOTE]]
[[B Alpha 1-1:1 (KK) REMOTE],[B Alpha 1-1:1 (KK) REMOTE]]
[[B Alpha 1-1:1 (KK) REMOTE],[B Alpha 1-1:1 (KK) REMOTE]]

Maybe this desync depending on locality of the vehicle is somehow responsible for this bug?

Yes, I was using UAV terminal, but not while in a vehicle. (AR2 Darter). I only gave Move and Land commands.