Updated this morning but seems whatever you did caused a few unwanted results.
Attempted pushback at KSEA and once the pushback tractor was set there was no request to release parking brakes. Instead the window popped up with pushback options. If actioned without releasing brakes there are the obvious unwanted results. If you release the brakes without being requested, you are requested to set them again. If ignored and you proceed with pushback, at the end of pushback and you set parking brakes it thinks you haven't done it and so the tractor never detaches.