Cut object pasted as bitmap

[LucaDC]

I cut an object (let's say a square or a circle), then pasted to the original location with ctrl-alt-v (actually I was moving it from a layer to another) and it became a bitmap. This was not the first cut-paste operation in the working session and all the preceeding worked fine.
Starting from then, every cut-paste, copy-paste and so on become bitmaps (pngs: every paste operation creates a .png file in the current directory, with the pasted object inside).
The resolution of pasted bitmaps is very low: few big gray squares (working with black paths). Unfortunately, I deleted all of them. I'll keep and post one if you think it's useful.
When this happens, I have to close and restart Inkscape to reget the normal behavior. I don't know what triggers this: after reopening the document the same cut-paste works fine.

Up to now this has happened only 3 times and always when working on imported PDFs.

Inkscape 0.46+devel, revision 20168, built Nov 10 2008 but also other two older devel revs.