TL;DR; I bought Apple gift cards from Coles. One of them says invalid. Customer support said it's been redeemed and declined to refund. I requested to escalate the case to higher management and waiting for response.
Update 1: 6/12 5.46pm I just got off the phone with Apple support. The tech support person confirmed that the card was redeemed and the person who redeemed it has used up the amount. They can't tell me when and the location it was redeemed though. They escalated it to another team who can see more information from the transaction to investigate it. They will get back to me by Wed next week.
When I asked why it was redeemed but the message says "Invalid code", they said the message varies, it doesn't necessarily have to say redeemed when it was redeemed!
The tech person also couldn't check if the redemption code and the security code are valid. They can only tell the security code on the envelope and the card are the same which is obvious from photos.
Update 2: 10/12
There are 2 similar cases reported below by @cfritz and @Ozjonno. They found out the last letter on the gift card was modified/rubbed out. Trying with the correct letter worked for them.
I took a deeper look. The last letter on my card was indeed rubbed out. It is a K instead of an I.
Unfortunately, it said already redeemed when I try with the correct letter.
https://postimg.cc/NLBWP7Gf
Long story:
I bought 3 Apple gift cards from Coles on 4/12, $500 each. I was well aware of the tampered gift card issue. Therefore, I checked the envelop carefully. It looked pristine, print quality was up to the standard, the seal looked untouched. I compared it with a few other cards, no difference.
I paid and they got activated at the checkout. Didn't open them right at the checkout because I felt I was blocking people. Stepped out, opened and redeemed them.
1 of the gift cards wasn't able to redeem. It said invalid code. I compared the security code on the envelop and on the card, they matched completely, and the inside card didn't look dodgy at all.
I came back in the store right away. The staff didn't do anything except telling me to contact customer support after 24 hours because it could take up to 24 hours to be activated.
Contacted customer support 24 hours later, they told me the card has been redeemed. I requested my case to be escalated and am waiting for their response.
On a side note, the code on the card I got still keeps saying invalid instead of redeemed. One of the security codes on the invalid card continues the ones on the other 2 cards (like D258X and the other cards have D256X and D257X), this made me think these cards were put on the shelf in the order they came from the supplier. If my card was really tampered, I'm really impressed with the attention to details.
At the moment, I'm just going to wait for their reply. However, if they keep saying it's none of their business that the card has been redeemed, who should I escalate this case to?
If the packaging was pristine then maybe the database for the cards has been compromised? Report it directly to Apple.