Well, I never said that YOU should stop using the grammar checker, only that I don't use it.
On my installation of Office 2016 Professional Plus (which does not have the Editor), "these sentence" is flagged as grammatically incorrect (when language is set to US English):

"Tests" is not flagged and neither is "if their fixes." I certainly agree that this is unacceptable, although the grammar checker has always had limitations.
I guess I'm really wondering what you expect the Community to do about the bug, when you know that we don't work for Microsoft. (Yes, it appears to be a bug, although we don't know if Microsoft will recognize it as a bug.)