I have also ran into a similar issue at times from the web under manage nodes sometimes when I select a node and unmanage it with unamanage until a year in the future. When I hit ok it often doesn't take for some reason. After doing it over and over a few times it finally takes and the node goes unmanaged. I assumed maybe there was a delay between hitting ok and the devices changing status in the database which I figured I could live with.
I had thought maybe it had something to do with the browser or browser version I was using. Do you get the same result from IE or firefox?