I wouldn't say this is a bug, it actually works as intended. Once you gain a Copper, you need to deal with the question if you want to discard the ones you have in Exile or not. Allowing to click on the card in hand (which has a blue color, not a green color as for playing) is thought of to lessen mouse movements.
But I understand that it may be irritating.
The usual way to deal with those irritating questions for discarding cards like Copper, Curse or Province from Exile is to activate the new "Privince" autoplay. Set it to "Keep bad cards in Exile", and you will not be asked that question on gaining Copper, Curse, Ruins and pure Victory cards (except Feodum and Farmland).
You can set/unset that option globally under Options/Autoplay, or per game by right-clicking on any of the mentioned cards.