Is there evidence of either yet? I’m not sure how you get from “potential” failure mode to “most likely”
They're trying to find the broken component; what caused the issue is usually a handful of things in space. Those are usually the top causes of component failure in that environment.
Although a relatively small part of my career, I spent some time working within the quality arm of an aerospace org. A lot more on propulsion systems, but both those and satellites are usually required to meet J-STD specs for electronic builds. After a few failure investigations, you become acutely aware of the dangers of prematurely jumping to conclusions.
So no evidence then.
I thought NASA no longer used lead-free solder to avoid this.
I think a lot of safety-critical systems even here on earth have exemptions from lead-free regulations because of this... but even lead can form whiskers.