Blocks placed in my world disappear if I exit and reenter the world

In certain parts of my creative world, if i place or destroy blocks in that part and then exit and reenter the world, the blocks destroyed get restored or the blocks placed disappear. My world has been around since 1.3.2 and this issue has happened since 1.7.2

What I expected to happen was...:
The blocks to stay where they were

What actually happened was...:
They disappeared when exiting and reentering the world

Steps to Reproduce:
1. I don't know how to reproduce this
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