Grammar checkers flag more than "invalid" grammar. Historically lots of arbitrary rules have been improperly applied to English to make it seem like a classical language. I like to enable checks for passive voice but completely eliminating it creates a stilted result and isn't practical, especially in technical writing. These tools are guide. I am free to ignore its advice when I know better.
... and yet you used an expression that seems unnecessarily idiomatic and parochial (British right?). I can easily imagine someone in Asia struggling to fit that literal expression to your intended meaning.