Passive voice in narration is ... strange most of the time. It kicks the reader out of the situation, like Out-of-body experience, only it is an Out-of-book experience.
"John was killed by the assassins, his body was later discovered by a cleaning lady, carried out by morticians and dissected by pathologists."
It is pretty hard to feel any emotions around this almost technically cold description. A sentence like this could have come from a police dossier.
I am not a friend of software validation tools in this regard, though. Capturing the essence of beauty in words isn't a solved problem.
"The assassins killed John, the cleaning lady discovered his body later, the morticians carried him out and the pathologists dissected him."
It is better, though not massively so. It's tighter. (Personally I'd break it into four sentences, but that's just me. I also would have used an Oxford comma, for that matter.)
The real problem is that the passive voice can be used for:
"John was killed, his body was later discovered, carried out, and dissected".
It's definitely punchy, and as the beginning of a thriller novel its vagueness could be exciting. But any place else you're left mostly with questions. It would be terrible in a newspaper account. And in an actual police report, it would sound like somebody was covering something up.
Slavs in general tend to struggle with "a/an/the" in English, because most Slavic languages lack articles. Out of the entire language family, only Bulgarian uses definite articles.
Articles feel unnatural to me, too. I got used to them, but in a similar way that one gets used to a prosthetic arm: you can do things, but it feels mechanic and distant.
Just like the passive voice, then?
*mechanical
And any writer who uses such a tool for validation is unlikely to be a good judge of writing to begin with.