The root cause of this bug is pretty obvious: The text of the dates such as "May. 04" when rendered in this font is slightly too long to fit in the window allotted by the display, so the interface tries to wrap it. "Mar. 04" fits, but "May. 04" doesn't because of character spacing.
This is aggravated by the inclusion of the dot (period) character, which is superfluous because May is not even an abbreviation. Actually, the dot should probably be omitted for any abbreviated months, such as "Apr" or "Sep" anyway for cleaner style. Just making that tweak, or increasing the size of the window control slightly, would probably fix this -- at least in English locales.
Since "May" is the longest month, the occurrence of this bug will likely go away at the end of this month. Hopefully it will be fixed before May 2012.