Why these sentences are incorrect?

I'm writing with Grammarly and the application says that I've written them wrongly. ( 2 Passive Voice Misuse)

It has been proven that studying a book is not the most reliable method. Words tend to be forgotten after not using them.

If I change the word "proven" to "confirmed" the application marks a mistake less. Why?

In a comment, choster wrote:

The short answer is that a software proofreader, at least as of 2019, as not as reliable as a human one, and that algorithms embed human prejudices and reasoning flaws (such as a blanket animadversion to the passive voice). Use so-called "grammar checkers" (and "spell checkers") as exactly that—a tool that suggests you check for the validity of a word to ensure you wrote what you intended—and never as an authority. Note that Grammarly's own blog acknowledges valid uses of the passive voice.