Once again, these are secondary sources at best. "people are saying that they heard somewhere that..." Are these using the same secondary source? I don't know. I haven't seen any evidence that it happens with save files.

The ~~second~~ third (didn't realize it was 3) link also mentions switching Proton versions too much can cause issues and uses a primary source, and I saw that review the other night while looking at reviews. That is much more trustworthy.

I'm not saying it isn't happening, but I don't trust what everyone says. I also don't trust that a user actually knows what triggered an action. The number of people I've seen say the saves are stored online because they don't understand Steam Cloud is proof that a lot of users aren't technologically literate enough to just take their word. With there being no first hand source, and potentially both of what you linked using the same secondary source, I still see no reason to believe this.

So, "How many testimonies do you need to consider it “evidence” as you say?" More than 0, which is what we're at right now.

Edit: Missed the first article when I clicked the links the first time. Even it says it can't verify the reports and it's just gathered from forums (and proceeds to not cite them). Any half decent journalist would verify it for themselves, but we know these aren't journalists, they're blogs that just repeat any drama they can find. Still only secondary sources at best with no citation, so nothing to be taken as anything more than the comment above saying "they heard it happened to someone."