Single Player - Entities Disappear Eg: item Frame, Minecart

Hey, I've been playing on my single player world for about 50 hours now.
I noticed that when I coming back from mining ((y:10) Far from my chest room) all of my item frame disappear. And when I am away from my minecart (in my mine) It desappear too... I just log out and log in and they are back.

not a big problem...