When you go inside an .iso image and copy folder using drag'n'drop all is well until you press arrow back button. Copying halts to an error message when you exit the .iso.

Tested a couple times, both tries resulted an error.

verified on my peg2 here

That's normal, because the iso is in fact a mounted volume internally and it's removed when the last lister referring to it is closed. Of course it would be better if copying would also keep a reference to it, but it's not the case currently.
So in short, just keep at least one lister opened on that iso during copy.

so, it's not a bug, it's a feature