Stuck on "Building terrain" leaving the nether

The game is getting consistently stuck on the  "Building terrain" loading screen returning from the nether back to the over world. I can travel from overworld to neither, just not back.

the first time this happened after leaving for 10 mins restated device and restarted game to find death screen saying i had suffocated in a wall. lost all items in inventory and items recently placed in a chest.

tried again, this time, restarted just the game and was sent to spawn in overworld.

Now don't want to try going to the nether again! Bit stuck....