Hi,
I don't see how product code could change themselves.
These "_copy" in the product code of the variants indicate that the product was copied with the "copy" button of the products listing.
When a copy of a product is done, its product code and all of its variants product code get a "_copy" along with some numbers added to the codes of the copied products as product codes need to be unique.
I don't see another way to get product codes with that "_copy" added than you clicking on that copy button at some point.