As far as I understand, it's kind of a feature, rather than bug, but it's very inconvenient.
When copying, nowiki tags get added to each line, double line breaks are removed and div block is added at the end. Those are visible changes, though and they could be undone manually.
However, there're invisible changes as well and some may brake template functionality. Please look at history of this page I had to fix to make templates work: https://ru.wikipedia.org/w/index.php?title=%D0%9A%D0%B0%D0%BD%D1%82%D0%BE%D0%BD_%28%D0%A2%D0%B5%D1%85%D0%B0%D1%81%29&type=revision&diff=90952690&oldid=90950606
Multiple spaces are throttled with something invisible..
Thanks!