Let's say you buy a fish with ich (not always visible as trophonts can harbor inside the gills.) After 3-7 days (dependent upon strain), the trophonts will drop off the fish and encyst into tomonts (basically, ich eggs). It is important to remember that there is nothing you can do up to this point to eradicate ich. Trophonts bury under the fish's outer skin layer and tomonts encyst, so both are protected from chemical and osmotic shock treatment. However, once the tomonts rupture and release free swimming theronts into the water that is when chemical/hypo works it's magic, as that is the stage of the parasite that is most vulnerable.
Ich is invisible to the naked eye in all stage of it's life cycle. Even trophonts on the fish, as the white spots you are seeing are just "exit wounds" caused by excess mucous building up around the insertion points on the fish. So, you have no idea at what stage ich might be in your QT. It can take up to 72 days for all the theronts to be released from their tomonts, although <28 days is more the norm at reef temperatures. And theronts are not all released simultaneously, ich's life cycle is never entirely in sync. Probably a defense mechanism to ensure it's survival.
So, if copper were to drop below therapeutic levels - even for just a second - do you see what could potentially happen? A theront could make it's way onto a fish and ich's life cycle starts a new. I realize the odds are highly improbable but .....