Okay, this is driving me crazy. I've been soldering for about 40 years, got plenty of skills and experience all the way down to SMD soldering with a microscope.... so why the question about soldering a stupid wire?
I'm repairing a splice for my external CCTV camera cable. The British weather took its toll.
Pair of double-insulated wires. One a twin pair for 12v power, the other coax for the video.
I cut away about 20cm from each end to avoid any oxidation / moisture etc, stripped the insulation nicely. Red +ve wire tins like a treat, core signal wire on the video cable tins like a treat. Straight away, no problems.
BUT: Can I get the ground on the video or the black ground on the power to tin? No sir. Solder just rolls off both.
Flux you say? Sure, I used flux. Proper leaded flux cored solder, plenty of heat, cleaned the wire, scraped it with a blade, tried mucking about with some offcuts.
Nope, the wire isn't lacquered. Checked a sample with a meter.
The ground wire on the power DOES look slightly darker than the +ve, but I've never had problems with a little oxidation before, bit of flux, abrasion no worries, even with the mankiest bits of wire.

So... what gives?
If I'd had trouble with ALL the 4 wires here then I'd be happy, they ought to oxidise equally...
It's almost like the fudamental CHEMISTRY of the copper has changed... but that's just silly.