To be honest I've not noticed it myself, but if you would rather Word didn't check your grammar you can turn it off by going to File > Options > Proofing.
If you want to get to the root of the problem, you could try contacting Microsoft about it?
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.