I have a MacBook Pro (15-inch, Mid 2010) and an LG 25UM64-S Ultrawide monitor. For 2+ years I had the Ultrawide connected by mini-displayport and got the full/native resolution of 2560x1080.
But just this morning, the max resolution dropped to 1080p according to the displays applet. The MBP said it had restarted overnight (some kind of update?) but otherwise nothing changed.
I just did another update and the MBP is now running OSX 10.10.5 but that didn't fix the problem.