Setting 2 spaces in the grammar settings only flags 1 space after a

period as an error with a green squiggly line. This does not "set" any kind of command in Word.

I have seen this problem reported, but I have never seen a true fix. Try turning off smart cursoring, smart paragraph, and select entire word options on the Tools/Options/Edit tab.

Workarounds are pressing F8 three times to select the entire sentence to the period. If you are using the mouse, pressing the Ctrl key, then clicking selects the sentence.