I found the problem (or better: I ran into the same problem) .. I guess it has to do with the European locale you are using .. with the US locale (starting the week with Sun) it would not appear.
The quick fix for this one:
class: CalendarPickerMonthlySkinAbstract
method: determineFirstOfMonthDayOfWeek
int lFirstOfMonthIdx = lDayOfWeek - lFirstDayOfWeek;
// fix
if (lFirstOfMonthIdx == -1)
lFirstOfMonthIdx = 6;
// fix
return lFirstOfMonthIdx;
(I looked into JFXtras for the first time today .. so I can't tell you "why" etc.)