"The single biggest problem in communication is the illusion that it has taken place." George Bernard Shaw
While following the rules drawn from established English usage, a writer has great flexibility in how to express a concept. Grammarly will never do that.
Voice refers to the subject of the content written. (i.e. What is your "voice" in this space?") What you write about is up to you. Don't let Grammarly change the way you tell it.
Tone refers to the way the content has been written and focuses on the way it has been presented. (i.e. What "tone" does your writing take on when conveying the message?") Don't accept a change that changes what you mean.
Beyond the essential elements of spelling, grammar, punctuation, and diction, writing style is the choice of words, sentence structure, and paragraph structure used to convey the meaning effectively. (i.e. What makes your "style" of weaving words together unique to you?") Don't give in and fix something that isn't necessarily wrong.