Yes, this is a bug.
I now have started to document my plans for the how the quiz should be changed to allow a whole lot of bugs like this to be resolved properly. See http://docs.moodle.org/en/Development:Changing_the_Moodle_Question_Engine (comments on that welcome, probably best in the quiz forum, even though it is incomplete).
In the mean time, I have plenty of other things to do, so am unlikely to work on this myself. If someone comes up with a patch to make things better in the mean time, then I will make time to review it, and commit it if appropriate. I can see two approaches in the short term:
1. Change the question type, so that general feedback is shown once the quiz attempt is closed. If you do this, you need to test both adaptive and non-adaptive mode. You also need to give a convincing argument, alongside your patch, why no one will mind if we change the current behaviour. (I have not thought it through in depth, but someone might like the way this question type currently works, and not want it to be changed. Perhaps this needs to be discussed in the quiz forum. Perhaps I am just being too paranoid.)
2. Change the working on the question editing form, to teachers understand the current behaviour of this question type.
I tried it today (May 28) and it seems the problem is still alive. Can you explain me why I am wrong?