[qwertyuiop924]

If you look closely enough at the spec, almost anything is conformant. But it's generally acknowledged that breaking everybody's code is a Bad Thing.

It's also possible that the test suite just doesn't check for this. In any case, the behavior is necessary for any application that polls against a set of fds of varying size, as the fd set may shrink to zero before expanding again, and and punishing the developer for not checking as special case that all other unixes handle automatically is pretty unfair.