Searching for closed answers should returns zero items due to only questions can be closed. But result is not empty and contains closed questions only:
I don't think it's a bug, or if anything, just a display bug, i.e. the "answers only" should indeed be removed as that option is simply ignored.
The advanced search operators got priorities behind the scenes, the
closed:1 operator happens to be on higher priority than
is:a so that's what being used.
Starting to check in advance for such "conflicts" is very complicated as there are great many combinations, and it's really not worth the efforts in my opinion.