When an owner window is iconified the system tries to hide all of its owned windows. This may appear to fail if "System Settings > Desktop & Dock > Minimize window into application icon" is OFF.

You can test this using the attached application. Launch it and you should see three windows. Each one is owned by the window below it. Iconify the lowest-most window. The two upper windows may do nothing more than flash. If you can't reproduce try toggling that system setting to ON and then back to OFF.

When a window is iconified the OS may or may not recreate all of the NSScreen objects (for some reason that system setting influences this behavior). This may cause the window to receive a windowDidChangeScreen: notification at which point it will try to reorder its children. If the child is hidden the re-ordering may unhide it and bring it to the front.

This was discovered by a system test in PR #1789, StageOwnershipTest.iconifyParentShouldHideChildren.