Minecarts randomly despawn and respawn after going far enough away

I had a minecart quarry set up that would transfer my items to a chest on the surface, then go back down, with another minecart ready to transfer me up and down. I was mining for about 15 minutes, and my inventory was full of ores and stones. I came back to find that both minecarts had despawned. I spawned in more and placed them. I loaded all of my stuff into the chest minecart. A few seconds after it went up, it came back down. With the speed of hopper transfers, that meant something was wrong and some of the stuff was broken. I reopened the minecart chest to discover- nothing! Nothing was inside. That could only mean that it was my old minecart chest. But how? They would have collided along the track. I took the minecart back up to the top, but immediately went back down, as my original minecart had somehow respawned and was at the top of the quarry! I hopped out before it went back down. But lo and behold, my chest cart- filled with minerals and precious stones, and mostly 7 diamonds and three emeralds, was gone! My stuff was entirely lost. So, I destroyed the extra minecart and walked down the steps, and found that both of the new carts were gone too.