It´s not a bug. you can store the actual canvas for further 2d sculpting stuff and rendering etc. Not sure, but it´s something with alt and double click or... dunno...but it´s not a bug.
Shift+S snapshots it to the document, ctrl+n clears it (you shouldn't have to exit edit mode and redraw your tool either, so you shouldn't have to do anything over again. Just ctrl+n and keep working). There was a glitch in one of the older versions, somewhere around r2 or r2p2 that caused this to happen frequently to a…