World keep changing, impossible gameplay

I just started a new game and I haven't played for maybe a year.
I install Minecraft Forge but no mod yet.
I am hiding in a sand hole because I am afraid of night. <yeah 😞>
I minimize the game so I can read a website while waiting for the day.
I dig a vertical lind to see if daylight is back
I reach the maximum digging distance (sand everywhere) (lets call it the "line")
After sometimes, sand fall and block my vision of the "line" (no idea if its fully filled)
I get bored and type "/time set day"
I wait some time to make sure mobs despawned
I dig the block that blocked the "line"
Bam. Everything bug but my character don't move even if under me, there's nothing.

GIF: https://gyazo.com/71412249e679cd953a0e1e662993ddba
(could not put it as an attachment since it's 40MB big)

MY QUICK GUESS: It keep generating a part of terrain (but sometimes it come back?) and make it repeat in both X, Y and Z directions.