NSWindowCollectionBehavior in Full Screen Spaces

The NSWindow collection behavior is supposed to allow for a window to appear in all spaces, even when another window is full screen in that space. However, I cannot get this to work as expected with any combination.

This should be as simple as:

window.level = .screenSaver
window.collectionBehavior = [
    .canJoinAllSpaces,

    .fullScreenAuxiliary,
    .canJoinAllApplications,

    .stationary,
    .ignoresCycle
]

If I have a window (Safari for example) which is full screen in its own space, run my app and change to the Safari window's space, my app's window is not visible at all; it will only float on top of windows in non-fullscreen spaces.

What am I misunderstanding?