In v0.6.0 Alpha, Creative and Survival (I have only tried Quartz, signs, nether brick, and smooth sandstone) blocks then pressing pause-> Quit To Menu-> Start Game-> The world you just left out of. will disappear as if they were never there.

*What I expected to happen was everything I had just placed to be there!
Describe what you thought should happen here

*What actually happened was the opposite, every thing as gone!
I had built a mansion out of quartz and about half of it was gone the next time I started that level

This happened to my mansion in creative, signs in Survival, and I was placing a lot of the blocks just added then quitting and starting again to see what would happen, and every time the same thing happened

The attached image is what is left of my mansion 😩