Not a bug, more of a weird ruling decided by konami. Quite a nerf but if you use an equip, deal with the monster you copied in some other way or even use murmur to get the field then set it after the copy etc. then the card's cool.
You guys don't get it. Copycat's effect triggers when it's finish summoning. So it initially gains 200 from Yami during the summoning process, then overrides that 200 boost with it's effect to be EQUAL to the monster, not GAIN. So no, it's not a bug, that's just how the mechanics work.
Yeah and it works incorrectly. It says the card is equal to the opponent's original attack. Why would it take the field spell bonus first, then add attack to make it equal to the original but then not consider the field? That is beyond stupid and makes no sense.