I'll make a couple of comments from experience.
A company mad some Berilium copper wafer probes hat turned nasty black at high temperature. They were looked at under an SEM and we tried to EDAX it. No luck.
The company used Sn96 or 60/40 solder to solder the needles. Aside: they changed from welding the ceramic to the spring support to 60/40 solder which created another headache down the road. they would solder with Sn96 for us, but could not go back to the weld process.
So, it was determined to be unremoved flux which the company would not fix. Our fix was to boil 10 minutes in Baking Soda and Electrolys Gold plate. They would not plate anything soldered, they did gold plate other parts of the assembly.
In a High School Explorer's post at Hewlett Packard when they did gold plated boards, you learned of the layer of Nickle that needs to be put down first. So, it's copper, nickel and gold for the PCB.
The same was done for electron-beam evaporated materials where I eventually worked.
You also learn with plumbing and silver soldering the importance of removing the flux. it's easier to remove when it's fresh.
When silver soldering AC refrigerent lines, you have to flow nitrogen in the inside of the tube while soldering. That's an extra step. When I put the AC in my car from a "big box", I used techniques borrowed from UHV (Ultra High Vacuum) No fingerprints anywhere. Lubercae all O-rings. You change gloves for a few reasons. You might have to change the glove because you contaminated it.
Try reading some medical rules with gloves that prevent contamination.
If you really care about purity such as i had to when sealing semiconductor synthesized materials in quartz, you have to etch the quartz in pure HF (Hydrofloric Acid). You don;t want to get near that stuff. A drop on your skin might kill you. The torch used was pure Oxygen and Pure Hydrogen. Again to reduce contaminants.
When working in High Vaccum environments, NO fingerprints are allowed. Then there are wacky thngs called virtual leaks that you have to eliminate when building the system. Screws (Vented screws) have to be used which have a hole down the center so you can pump out the air between the threads.
I've seen plenty of PCB board damaged when the flux wasn't removed.
For soldered pins, I generally used a resistance soldering tool, so the pins would not get solder on it.
I had contamination issues with brand new solenoid valves. brand new valves would buzz if operated off of dry air and 24 VAC. As received, they had to be taken apart, cleaned and re-assembled.
The extra steps were required and I never had issues.
So, for your cables, resistance soldering and flux removal ought to have worked.
For cables that I made at work for use at work, I generally used gold flash pins. These were typically circular connectors with 4, 9, xx and 37 pins.
For electrical connections that used screws, loctite 222.