Mobs don't spawn when more than 4 players join world

After downloading "Clarity" textures pack, our world don't spawn any mobs when exceeding 4 players (we usually play 5 players on xbox + 1 player on pc). It gets more noticeable after more than 5 nights without sleeping on the game, phantoms never show up. As soon as 2 players disconect, phantoms suddenly appear as well as the rest of mobs. Sometimes it also happens when 2 players go into the nether instead of disconecting.