I think I know what you mean. I get it when I get quite a dense mesh, and I'm trying to work quite quickly when my computer is starting to struggle. My ztool will drop to the canvas and I'll get a duplicate still in edit mode, same scale. I think there is a 'quick click' command in z, which I always think is slightly odd, I.e 'quick click in the canvas', so whenever this annoyance happens to me I assume that that's what I've done, and it's that that has dropped the tool to the canvas. It never leaves edit mode. I'm not certain that I'm right about that command, but I think that's what it is, and therefore it's an annoying command, rather than a bug.