Two spaces after a period is an error →
source: theverge.comTom Warren:
“Microsoft has settled the great space debate, and sided with everyone who believes one space after a period is correct, not two. The software giant has started to update Microsoft Word to highlight two spaces after a period (a full stop for you Brits) as an error, and to offer a correction to one space.”
You can easily disable this change but it will be the default moving forward. I haven’t used two spaces after a sentence (or double spaced sentences) since I was in high school. This habit makes sense when teachers utilize the additional white space for making corrections but outside of that limited use case there is no reason for it.