The trouble with using bugs is that most of the work for fixing them is in isolating just what's going on and where the problem is, and there's no telling how long that will take until you do it. A bug on a new codebase could be anywhere from a few minutes to a few months.
In particular, if you don't know the codebase, you're going to be guessing on places to look. If you're lucky and guess right, you may spot it right away. If you guess wrong, you could spend a lot of time diving around irrelevant details. There is some skill in guessing well and not spending too much time on things that don't give any progress, but there's still a lot of luck too. Because of all that, it doesn't seem like a good idea to assign a bug that you haven't already essentially fixed.
Maybe it'd work to just timebox the task, i.e. 'work on one bug for an hour', along with 'document everything you find and learn' (and add it to the public issue). I'm a habitual documenter and I could imagine something like those 'papertrails' being a useful way to evaluate someone's thought processes (and written communication skills).
In particular, if you don't know the codebase, you're going to be guessing on places to look. If you're lucky and guess right, you may spot it right away. If you guess wrong, you could spend a lot of time diving around irrelevant details. There is some skill in guessing well and not spending too much time on things that don't give any progress, but there's still a lot of luck too. Because of all that, it doesn't seem like a good idea to assign a bug that you haven't already essentially fixed.