Confusing onMonthDay option

[gglnx]

If you create an event on the last monday of April (27th) and select the option to repeat it monthly, the dropdown says "Last Montag" (our installation is in German). After saving the event the dropdown options changed to "Fourth Monday" and the dates for June and August are not the last Monday (because they have five Mondays).

If you use the 30th of March as starting date (which is a 5th Monday), the dropdown says "Last Monday" after saving and all repeating dates are correct on the last Monday (ether the fourth or fifth).

We have events both with $Xth weekday and last $weekday of the month.