A pre encoded tag cannot be rewritten
Establish which of three states you received before contacting anyone. Unlocked with unwanted content is a nuisance you overwrite. Password protected is recoverable if the supplier holds the key. Locked is a loss, and the wording on your order decides who carries it.
Three states, one symptom
Unlocked, with content you did not ask for. A demonstration address, a supplier page, or leftovers from a previous job. Harmless: you overwrite it and lose a step, nothing more.
Password protected. The tags read and refuse writing. Recoverable if whoever configured them will give you the key.
Locked. Permanent, unrecoverable, and the tags are only good for whatever they already say.
All three present as a tag that will not accept a write, which is why the first move is to find out which one you have rather than to complain.
What to establish first
Read several tags from different parts of the delivery. A mixed shipment where only part is affected changes what you are asking for.
Note the proportion. All of them, half of them and three of them are three different conversations.
Photograph the reading and the packaging together, with the date. A dated record of what arrived makes the exchange short.
And stop writing. A batch you have started writing is harder to return, and any tag you lock yourself becomes your responsibility rather than theirs.
The conversation with the supplier
Ask which state was intended. Suppliers pre encode tags legitimately and the mistake is usually that the order and the production did not match.
If they are password protected, ask for the key. That is often the fastest resolution and it costs nobody anything.
If they are locked, ask for replacement rather than a credit, since you presumably still need tags.
And ask what changed, because a delivery that differs from the last one points at a substitution that will happen again on the next order.
What should have been on the order
The words blank and unlocked, in writing, unless you ordered encoding.
If encoding was ordered, the exact content, and an explicit statement of whether tags are to be locked. Left unstated, that decision falls to whoever runs the machine.
The chip reference and the NFC Forum tag type, so the delivery can be checked against something rather than against a memory of a conversation.
Adding those lines costs nothing and converts this incident from a dispute into a procedure.
Frequently asked questions
Is pre encoded content a problem?
Only if the tags are locked or protected. Unlocked demonstration content is harmless because you overwrite it.
Can the supplier unlock them?
No. Locking is permanent for everyone including whoever set it, which is why replacement is the only remedy.
Should I write the rest before complaining?
No. Stop, sample several tags, and record what you found. Writing further weakens your position and may lock tags yourself.
How do I stop this happening again?
Put blank and unlocked on the purchase order, and read a sample from every delivery before writing anything.