New Airtag already connected to another Apple ID

Ok, even if someone physically removed this tag from your possession, unless you did not initially paired it with your Apple ID, then yes, they could pair it with their's. However, why they would return it just doesn't make any sense even if was to play a cruel joke.

Since you got it less than a year ago, it is still under Apple's hardware warranty, but that just covers manufacturing defects, which would not be the case here.

Unfortunately, there is no way of resetting it to remove their ID. Apple done this intentionally so that, if you had paired it, no one else could use it. In this case, the reverse has happened.

You can certainly contact Apple Support directly about this, but I'm pretty sure they will only provide you with the same information as this support article:

• If an item or device is connected to another Apple ID - Apple Support

You said you purchased this from the "official Apple website". Does this mean that you went to Apple to make the purchase?

There are only three reasons, that I can think of, that this issue can occur:

1. The tag is defective.
2. You purchased a previously used & re-packaged tag. I would think this to be a rarity if you got it directly from Apple.
3. With the way things are going on these days, the original tag was swapped out. This could have happened duting transit when you first purchased it, or happened afterwards. (Sorry, really going out on a limb here.)

You're very welcome! I'm sorry I couldn't help you with a solution.