Why hostile mobs start to spawn then all of a sudden they just vanish? Minecraft 1.10.2

If it was not a mod pack, I would say that hostile mobs naturally despawn afer a period of time. If you move to far away from the mobs, they will despawn from that area and spawn in a dark areas somewhere around you. This point is made when testing mobs in a fortress and overworld. In the overworld, I have tested the theory that mobs despawn after you die. This is false. If you die near the spawn area or bed (if you have one), you will respawn and the mobs are still there. However, lets say you died in a fortress that is 2000 blocks away from where you spawned. After awhile of you not being near there, those mobs automatically despawn from that area just so they could spawn where you are. As for the never spawning at all, there are only 2 options. The first is that the light level is too high for them to spawn, and they instead gather in underground caves. The second is that, ultimately, you glitched your game This sometimes happens with save files if not properly shut down, and you may need to modify the file or just create a new world.

Part of the problem with answering your question is that it is unclear exactly what your question is.

"...hostile mobs will start spawning at night then all of a sudden vanish with out a trace within 2 days (Minecraft time)"

Your opening statement sounds like normal typical behavior. Hostile spawn and despawn based on ridged game rules. They only spawn further than 25 blocks away from the player, and less than 129 blocks from the player. once they are closer than 23 blocks to the player they cannot despawn. If the player moves more than 82 blocks from a hostile mob it will despawn. One of the ways to clear a small island for example, is to row away from the island then rapidly return before many hostiles can respawn. If the island is smaller than 26 blocks across nothing will be able to spawn on it while you are on it.