While exploring new areas of the map, the game works normally.

When making a return pass through an area that was just explored, the game "despawns" the world in certain sections!  The smallest looks to be an exact square with maximum height from bedrock all the way up.

However, there have been other much larger chunk despawns that aren't perfect squares.

Provided here are 3 'screenshots' of two despawn points in the ocean directly next to each other:

(1) Eastern despawn with two maps (Zoom 3/4 and Zoom 0/4) hand-in-hand to see what it looks like on the map.

(2) Same eastern despawn, no items in hand to provide better view.

(3) View (facing south) of the two adjacent despawn chunks.

These photos were taken not on the return pass, but on a bonafide return trip in order to document the issue, or see if it was naturally resolved.

By despawn, I literally mean all blocks have been removed.  Going into it you fall straight down to the bedrock and die.

This is a Survival world (Achievements enabled) on Normal mode with two-players playing local on same console, non-realm/server.  Natural texture pack, save file is about 185MB.  I'm always signed into the Xbox/Microsoft/Minecraft accounts on main menu before loading the world.

I will provide more photos and details of other locations upcoming, especially the worst one.

The ONLY blocks that don't despawn are source blocks for lava and water.

This despawning bug has left us terrified, especially without Jagex readding the Quit Without Saving feature, we have no control when our world is destroyed like this!