Cloning sufficiently large (laggy?) areas has old blocks affect new ones

Verification builds:

1.5.0.4 beta

Summary:

Cloning in a pretty large area, one large enough to cause lag atleast, makes the OLD DESTINATION blocks affect NEW DESTINATION blocks.

Impact:

Gameplay breaking bugs-> blocks creating or destroying when not intended. Can be hard to catch for procedural like maps where the destination contents isn't always known. Not quite blocking soon to be shipping content, but a work around will be performance costly.

Repro Steps: (updated 6/7/18)

1. Create a new map, flat. Creative.

2. Execute the following commands:

/tp @s 345 4 8
/fill 339 4 5 315 10 -15 snow
/fill 339 20 5 315 20 -15 lit_pumpkin
/clone 339 16 5 315 40 -15 315 4 -15 replace force

Observed Results:

Snow golems appear.

Expected Results:

No snow golems.

Screenshots/Videos attached: No

Notes:

If you do the repro steps with only a one block column instead of a large area, you do not get any snow golems.