First appearance when I was riding on a boat in the overworld terrain didn't load I started falling into the void, Save & Quit -> Reload world I was with my boat at the bottom of the sea drowning, had to destroy boat manually to get it back and up to the top it was not rising up by itself.

Second appearance after creating a nether portal going throw world didn't render in so I Save & Quit -> load world again and I sunk through the floor and almost died in a lava lake, if it wasn't for Open to LAN I'd have lost all my stuff without any way to stop this.