I have no issues with Grammarly other than the ones you listed. I've had the same experience. When I was an Editor I would PN submissions and a lot of times I'd get the same response.

"Grammarly didn't catch it."

What had they missed? Medium and not medium. 11 times or more throughout the submission lower case "m". To which I'd have to remind them that it's a tool to assist and not to do the writing for them.