Well, about the only thing it can be is a fault in the fret material itself.
It's obviously not the strings, or the wood, or your fingers that are at fault.
That just leaves the frets and what kind of material they were made from.
If these are copies, then the manufacturer has probably cut corners in the costs of materials. That's why copies are less expensive than the real thing.
You basically get what you pay for. The guitar manufacturer is probably buying the fretwire from a bulk, overseas supplier who's using filler material in the pure metal so he can make more frets for less money.