The problem is following : we play on a server(me and my friends) and all is going well. But after the server is online for about 3-4 without restart the mob spawns start to reduce and eventually they become minimal and you can walk in the dark and don't see a single hostile mob. It is the same in the End too not only in the Overworld. The Ender Farm stops working because no endermen are spawning. The admin actually took it one step further and loaded a new world in which he was AFK for 12 hours to see if the mobs will stop spawning there too and eventually, they did stop spawning. So there is something messed up with the code I guess. Also I want to add that when we restart the server the mobs start spawning as they should and of course after a while they stop again.