Yes, I do. I have cleaned bores with just water or water and soap or cleaning solution and any number of patches. Finally they will come out perfectly clean. Everything looks clean and shiny with the bore light.
Then put a good squirt of WD40 down and a few more patches and eventually they too come out clean.
A good gun oil copiously applied, put away, taken out weeks later and the dry patches to wipe the bore come out stained (no, not rust!).
Seems the real test of cleanleness is to be able to push a very tight, dry cleaning patch all the way down the bore with no resistant, no jamming. And to feel smooth all the way down, no drag, no restriction, to the bottom of the breech and back again. I can only get that by the jusdiscious use of a bronze brush, particular at the breech end. Always a surprise what continues to come out of a 'clean' bore with that.
What causes the hard to remove fouling? My theory is the burning of both the powder and lube together can leave a hard, possibly greasy 'varnish' on the walls that builds up over time.