This is a nasty bug. Makes Sigil almost useless, at least for me, inspite all the good new features.
But there is a work-around. CMD-C for copy works, then use right-click into search or replace and choose paste.
This takes a little more time but it works.
BTW CMD-V into the Save As field does not work either, but same work-around.
I hope that also works for Win 8.