I've observed that mobs don't move when they are too far away from you, usually out of render distance. If you have short render distance, you can notice it easier. I don't know if this is intended ot not.
To reproduce:
1.) Set render distance to 2 chunks(not necessary, just makes it easier)
2.) Spawn a mob that tends to move a lot(Like slimes, silverfish or endermen)
3.) Travel far enough away so the mobs are almost in the "fog" of your render distance
4.) Notice the mobs stop moving
I've even tested this with the wither boss and ender dragon. Now I know why the ender dragon stops flying when it gets too far away...
Notice in the pictures how all the slimes and magma cubes stop jumping, and the endermen stop teleporting.