Spawn Chunks

Against popular belief spawn chunks DO exist on bedrock edition. These chunks around a given player's spawn point and are there mostly for performance reasons. If you have ever went into a portal and came back you will know that the transfer from nether to overworld is much faster than the opposite. This is because of spawn chunks. This can help you load those chunks to the ticking status much quicker than if they were fully unprocessed. These chunks form a rounded 11x11 square of unloaded chunks and of course bordering them is a rounded 15x15 square of locked chunks.

Where do they come from?

Spawn chunks are a consequence of 

void Player::updateSpawnChunkView();

which is called by these functions to move the spawn chunk view region to where you last set your spawn.

void Player::setBedRespawnPosition(BlockPos const& bedPosition);
void Player::setRespawnPosition(BlockPos const& inRespawnPosition, DimensionType dimension);
bool Player::setSpawnBlockRespawnPosition(BlockPos const& spawnBlockPosition, DimensionType dimension);

(Visualization by @kiwixite on discord)

These chunks will become unprocessed on server/world shutdown until loaded again. After which they will stay persistent across that session.