I know it shouldn't be there so I don't let it stay there for fear it would have moisture / fouling trapped under it.
For example, I ran across some bags of old chilled lead shot, which as you know is pretty soft due to very little antimony.
When I ran lengthy pattern testing sessions, then cleaned the bore, I noticed faint gray traces of lead.
(Don't recall getting that using magnum hard shot)
Next range session after pattern testing was finished, I fired a half dozen tight fitting PRBs and the tight patch weave caught / dragged all the lead residue off the bore walls.
I don't shoot bare round balls, can't comment on that.