The bug
I have created a moving machine that moves a relatively large structure (16x64x16) 1 chunk (16 blocks) at a time by using Command Blocks. This machine worked perfectly as intended in 1.14. However, whenever I attempt to use it in 1.14.3, the Command Blocks that move the structure (which are inside the structure themselves) automatically run again once the structure is cloned, creating a perpetually moving machine that lags out my game. My theory is that if a powered Impulse Command Block is cloned, the cloned version automatically runs again.
How to reproduce
Enter the following command into an impulse command block:
clone ~ ~ ~ ~ ~1 ~ ~ ~5 ~Place a redstone block on top of the command block
→ ❌ The command block is cloned repeatedly, up until the build height limit
Expected result
The command block should only be cloned once, as the resulting copy is already in a powered state when placed.
Also affects 1.14.4. I also have similar machines that worked in 1.12.2 and all 1.13 versions and 1.14.0. I went through the snapshots one by one verifying exactly when it started. The first version with the issue is 1.14.1 Pre-Release 1. Something changed between 1.14.0 and 1.14.1 Pre-Release 1.
Easy way to repro:
/setblock ~1 ~ ~ minecraft:command_block[facing=down]{Command:"clone ~-3 ~-1 ~-3 ~3 ~2 ~3 ~-3 ~20 ~-3"}
/setblock ~1 ~1 ~ minecraft:stone_button[face=floor]
press the button, it will make multiple copies of the platform going up until it hits the height limit and you will see multiple clone outputs