Zoglin "IsBaby" entity tag cannot be changed from true to false (1b to 0b) with /data

Can confirm in 21w03a.

Can confirm in 21w07a. Here are some steps to reproduce this issue:

Steps to Reproduce:

• Summon a zoglin using a spawn egg.

• Run the following command to convert the zoglin into a baby:

/data merge entity @e[type=zoglin,limit=1,sort=nearest] {IsBaby:1b}

→ ✔  Notice how the zoglin is now a baby.

• Run the following command to convert the zoglin back into an adult:

/data merge entity @e[type=zoglin,limit=1,sort=nearest] {IsBaby:0b}

→ ❌  Notice how this does not work.

Can confirm in 21w15a.

Can confirm in 1.16.5 and 1.17 Release Candidate 1.

Can confirm in 1.17.

Can confirm in 1.18.1.

Can confirm in 1.18.2.

Can confirm in 1.19.

Can confirm in 1.19.2.

I can confirm this issue in Minecraft 1.21 pre-release 1.

Code analysis (Yarn mappings)

The following code in the ZoglinEntity class manages the babiness of zoglins:

@Override
public void writeCustomDataToNbt(NbtCompound nbt) {
	super.writeCustomDataToNbt(nbt);
	if (this.isBaby()) {
		nbt.putBoolean("IsBaby", true);
	}
}

@Override
public void readCustomDataFromNbt(NbtCompound nbt) {
	super.readCustomDataFromNbt(nbt);
	if (nbt.getBoolean("IsBaby")) {
		this.setBaby(true);
	}
}

Notice that the zoglin becomes a baby if the IsBaby field within entity NBT is true. There is no behavior for making the zoglin not a baby if the IsBaby field is false.

Compare to the same methods in other entities with a boolean babiness status (zombies and piglins), where the IsBaby field controls babiness regardless of being true or false:

@Override
public void readCustomDataFromNbt(NbtCompound nbt) {
	super.readCustomDataFromNbt(nbt);
	this.setBaby(nbt.getBoolean("IsBaby"));
}