Perhaps not broken, but documented inadequately. In http://developer.android.com/training/implementing-navigation/temporal.html there’s a simple guide on how to set up back stack navigation when opening an app e.g. from a notification using a “deep link” to an activity that is not the topmost one in the app, so that the user can navigate upwards in the stack by pressing the back key.
Having wasted a couple of hours trying to make this work in LoanShark, I finally noticed a warning in LogCat:
W/ActivityManager﹕ Permission Denial: starting Intent [...] not exported from uid 10131
What the documentation forgot to mention is that you need to set the activity you want to jump to from e.g. a notification as exported. I added the attribute and encountered no more problems. The weird thing is that it did actually sometimes work even without the exported attribute… strange indeed. But now it works 100% of the time, fortunately.