I'm pondering this too, and I'm willing to be wrong on it, but I would have said that you can't be disappointed by a movie without having had some sort of raised expectation about it. If you thought it would be rubbish you wouldn't be disappointed.

"Dis" always suggests to me some sort of removal, rather than just the absence of.

My dislike of mornings does not, I hope, suggest I ever liked them. A disability isn't always an ability that's lost. etc.

So many exceptions, I sometimes wonder how others ever end up learning good English. :)