Note that this is not a dupe of MC-269830 bc while that bug needs the player action, this one don't.

If you a command block that repeatedly modify a or some data of a mob, it makes that mob unable to turn its head left and right, it can only turn their head up and down, and if that mob is leaded, the lead is unbreakable. Breaking the lead and the fence does not work, only killing that mob or turn off the repeat command block works. Some modifications are:

• "Health", "Fire", "CustomName" and "HandItems" on most mobs,
• "Color" on sheep,
• "CollarColor" on cats and wolfs (dogs),
• "body_armor_item:{id:"minecraft:air"}" on wolf, horse, llama and trader llama,
• InWaterTime, DrownedConversionTime for all type of zombies,
• ConversionTime for zombie villager ... (there're probably more that I cant find yet)

Step to reproduce:

1) Summon a mob that u want to test, stand around that mob and wait -> The mob looks at you normally.
2) Place down an repeating command block, right click on the command block, then type:

execute as @e[type=minecraft:<that-mob>] run data merge entity @s {<entity-data-compatible-with-that-mob> : <any-valid-word-phrase-or-number-that-you-want>}

, then set to "Always Active" and then click "Done".
3) Take a look at that mob.

Observed result: That mob weirdly looks up.
Expected result: It normally looks at u.

4) Leash that mob into a fence, then break the fence.

Observed result: The lead breaks
Expected result: No, it won't