Don't worry about the disappearing thing, sometimes if you build too fast or dig too fast the chunk you are disappears but if you place one more block or something else or if you dig once next to the edge of the gap it all reappears. Relogging also makes it reappear too ofc
Awesomely built :O
I had this issue of disappearing blocks again, but this time without even building, I was just walking around
Somehow its persistent and won't reappear So the city's insurance agents will contact you soon