What I expected to happen was...:
Managed to fly to the edge of a chunk in creative and out of curiosity I placed a a few blocks into the unspawned chunks. I expected that once the new chunk spawned it would simply deleted the blocks I had placed.

What actually happened was...:
Copies of the block structure I had made were copied to unspawned chunks. I was able to add more blocks to the structure and "grow" the structure. Even build back into spawned chunks. Back at the original space, the structure was deleted.

Steps to Reproduce:
1. Get to the edge of a chunk that hasn't created new neighboring chunks
2. place blocks into the unspawned chunk
3. wait for new chunks to be created