This is a really interesting bug where you suddenly become invulnerable in a server for no reason: you can't get damaged by enemies, fall damage, or your own gernades.
However, to prevent being called a ɥɐɔʞǝɹ or bug abuser, there are two ways to fix this in game. The first is to simply leave the server and rejoin, but you'll lose all kills, assists, coins, ect. However, I found the second method: entering vehicles. When you enter a vehicle (can be either jet or car), the invuln gets removed, and you can now be killed.