When you are looking at the calendar and it shows the moons, clicking on them to get the details on their phases shows different phases to those shown on the calender.
Go to the calendar and click on the moon phases displayed. The pop-up will not reflect the same phases.
The Calendar.
Here's an interesting note. If I have an event on that day, and click on the event, the moons display correctly.
Your results may vary, but on my world (which also has this irritating issue) it seems that every month copies the first. For example on my homepage the 15th of the first month is correctly a Waxing Gibbous, and every other month on the 15th is also marked as a Waxing Gibbous on the popup only, regardless of if it's correct or not. The same is true for the 1st (new moon) and every other date I checked.
Hey, thanks for reporting! We are currently working on the next version of Calendar which will resolve this issue, please keep an eye out for an update soon.