I believe the world was created in world 1.13 pre10, not 100% sure. Haven't strayed very far from my build area and haven't done any real exploring. Not sure when it happened but I just found that a number of chunks in my area have changed. It's like they've been reset but the don't match the surrounding chunks. Part of a wall I had been creating is deleted and has been replaced by a mismatched chunk.

Actually looks like one of the chunks has moved to another location. In 16.39.45.png there are stone bricks in the chunk. It looks like that's the chunk that's supposed to be in 16.16.39.png. Screenshots of those two chunks added with debug screen info.