Well, that was my best guess. Maybe since the offset in your frontmatter is set for UTC (+00:00) it's causing a problem? You could try setting the timezone to UTC or removing the timezone if all of your articles are set for +00:00.
Sorry this is kind of a shot in the dark. Maybe one of the devs can help.
Yes, removing the time part of the date-time string is a workaround (so I'm not stuck or anything).
The issue that I wanted to point out here is that "middleman article " for a middleman blog creates a filename with only the date (without the time) but adds a YAML frontmatter with the date and time strings which does not build out of the box.
The timestamp isn't even mandatory - it's just in the template so people know they can include it if they want multiple posts from the same day to sort correctly. We should figure out how to generate that timestamp in the specified time zone...