I don't know if it is a bug, but here it is:

If you set the "uses:"-tag of a custom villager trade to 1 or higher, it will unlocking new not custom trades, like it does normaly. It should not, because it should only do, if you're trading "new" trades. New trades have (custom or not) a "uses:"-tag of 0. So if you set it to 1, it isn't "new" anymore.