Steps to reproduce

1. Player 1 relogs in the nether.
2. Player 2 loads the overworld, either by relogging there (saved & quit before player 1 relogged) or by relogging in the nether and travelling to the overworld.
3. Player 1 goes to the overworld at the same location as player 2.
4. Player 2 travels away from player 1 to a distance greater than player 2's render distance + 5 chunks.

Expected result

The game continues to simulate chunks around player 1.

Observed result

The game stops simulation in chunks that are > player 2's render distance + 5 chunks away from player 2, for all entities that existed when player 2 loaded the overworld, except for player-launched projectiles which fall through the world into the void (MCPE-141539). Player 1 is still able to move and interact in some ways, but many entity updates get postponed until player 2 returns to within the threshold distance.

The attached video MCPE-147391.mp4 demonstrates this. White and blue blocks mark the Z = 47-48 chunk border. Steve's render distance is 6, and when he crosses the Z = 223-224 chunk border the entities on the left stop and tridents there fall, then when he crosses Z = 239-240 entities on the right stop and tridents there fall. MCPE-147391 test world.mcworld is the test world used in the video.

Simulation distance reaches to 14 chunks, but only for a single player. Crossing into the 15th chunk for the one player while two are on Realms activates simulation. That player backs out of the chunk by one block, other player witnesses simulation pausing. Villagers stop bouncing in water, animals stop moving, other player without simulation cannot push minecarts, hoppers stop transferring items. Sounds remain active for nearby player while simulation is suspended. Simulation-active player leaves Realm, simulation resumes for sim-locked player, and effect is transferred to the newly logged in player that previously activated the simulation at significantly greater range than the 4 chunks applied by Realms worlds. Root cause unknown. Both players logging out simultaneously resolves the issue, but can reoccur.