Mobs do not travel through Portal in Multiplayer when 2 or more players online

I have a Portal in the Nether where mobs are being pushed into. When I am alone on the Server, the mobs are traveling to the overworld portal as expected. If there is another player online in the overworld (very far away from the chunks where the mob portal links to) the mobs do not travel to the overworld but rather dissappear.