During my game i was confronted to a bug including one of my replicant falling through the ground.
What happened is i was connecting 2 water holes by diging beetween them and one of my duplicant was on a block right on the ground level. But when another of my duplicant broke the block my duplicant fell with the falling animation and everything and glitched through the ground finding himeself entombed. I can't really explain the setup anymore i wasn't really paying attention more than that.
All i know is in the message. But i guess you need a duplicant one block above the ground on another block and when you dig it the replicant fall though the ground.