1.12 Single player world rollbacked

Played a new SSP since the 1.12 came out, built a mob farm underground, (double skelespawners), logged in on the 28 of June, and the world had either been rollbacked, some chunks were replaced with stone and ores, or chunks have been refreshed/Remade. If anyone has encountered this let me know, might just be a one off.

Cheers
WFD