Items disappearing from chests

Had two diamonds in a chest and went caving and mined 10 more. Went back to my base and put the previous 10 I had found in my chest so I had 12. Then went back to explore and found 1 more and when I opened my chest the previous 2 I had were there and the 10 that I had were gone. When I had brought the 10 back up I rearranged my chests and when I opened it and found it missing my item placement was just as it previously was before I had placed the 10 in there. Why did they disappear?