BYSETPOS takes every month instead of the whole set

[lksasr]

E.g. the following RRULE should result in one event each year (as there is only one position, which should be taken by BYSETPOS), but has two occurences in 2025 (30.11. and 28.12.) – or zero occurences in 2028:

RRULE:FREQ=YEARLY;BYDAY=SU;BYMONTHDAY=1,2,3,4,5,6,7,27,28,29,30;BYMONTH=11,12;BYSETPOS=2
(Semantically it should be the first Advent)

It seems to take the second event in every month (11 AND 12), but should take the second event of the whole possible set of dates. In difference to e.g. "BYDAY=2SU".