Something grew in it

A dry powder is one of the safer food-adjacent products there is, right up until moisture reaches it. Then it is a growth medium with a large surface area.

Dry powder is safe until it is not

The low-to-zero moisture content of these formulations is what gives them a long shelf life, because microorganisms cannot live without water. That is the whole of the protection.

It is not a preservative, not a process step and not anything you added. It is an absence, and absences can be filled.

How water gets in

Three routes, in order of likelihood.

The customer. A wet scoop returned to the tub, a damp kitchen, a lid left off, a tub stored above a kettle. This is by far the commonest and it is outside your control except through instruction.

The package. A seal that is not a moisture barrier, on a hygroscopic powder, over months.

You. Filling in humid conditions seals room air against the product, and a raw material that arrived damp brings the problem with it.

What it looks like

Clumping first, which people read as caking. Then discoloration, then visible growth, then a musty smell.

The important distinction is that caking from moisture and mold from moisture are the same process at different stages. A tub that has caked is a tub that has taken up water, and treating that as cosmetic is a mistake.

Preventing it

A real moisture barrier: induction seal or foil pouch rather than a friction lid, and a desiccant where the formula justifies it.

Dry filling conditions, sealed bulk storage, and checking incoming material for moisture rather than assuming.

And clear instructions to the customer: keep it sealed, keep it dry, keep the scoop dry, do not store it in a bathroom or above a hob. Those sentences do more for this failure than anything in your process.

References