Dead soldier's descendant goes looking for the details of his death, finds descendant of another (surviving) soldier that was good friends with him. That surviving soldier's descendant had documented the stories that surviving soldier had told, including the occasion of the first soldier's death(and their friendship). The death happened due to getting hit by a shell during an offensive.

It's an engaging story, which I'm of course not doing justice to here, but the word "mystery" is just absolute clickbait.