If there is a difference in how Grammarly and Word interpret your sentence then I would be inclined to accept Grammarly as the more likely to be correct.

Both Grammarly and MS Word have algorithms set to identify particular grammar patterns and things they flag can be acknowledged as potential issues. However, you are the author and owner of your documents and as such, you should be the one to make the final decision on what is right for you.

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Andrew Lockton
Chrysalis Design, Melbourne Australia