I’m working on making DTCoreText iOS 6 compatible and when implementing line heights I found this problem. Naturally I filed a radar.
This is one of many shortcomings of UIKit’s support for attributed strings. A workaround for this was described as having to omit font attributes from the attributed string. So you can either have multiple fonts and unchanging line height or only the font you set on the font property of UITextView.
Other people are having the same problem, as outlined by this question on StackOverflow.
Filed as rdar://12863734