Last day of month is wrongly converted to VCalendar

[cscholz]

If a set a recurring event in outlook to the last day of the month, caldav converts this event as [...] RRULE:FREQ=MONTHLY;COUNT=12;BYDAY=MO,TU,WE,TH,FR,SA,SU;BYSETPOS=-1 [...]

This produces a daily event instead of a one time event on the last day of the month

If the recurring event is set to one specifc day of the week (e.g. Sunday) for the last day of the month it looks like

[...] RRULE:FREQ=MONTHLY;COUNT=11;BYDAY=SU;BYSETPOS=-1 [...]

is this issue a convert issue or does carddav not support the repetition (last day of the month)?