It was bad, but not all that surprising. The Colts have been in a bunch of one possession games this season, playing up and down to the level of competition. Throw in the fact that the Dolphins have been mostly competitive since September, the Colts rolled with Adam Vinatieri even though he's no longer reliable, T.Y. Hilton was out, and they were playing with Hoyer at QB, and it's not that big of an upset. Miami isn't as bad as we thought. Check the game logs. Since they switched to Fitzpatrick at QB they've still been bad, but not historically so.
The Colts were 10.5 point favorites earlier last week and if I was a betting man I would have bet big on that one.