Same issue. Deleted the lessons to make room on my drive. When I tried to re-download through Garageband not only does it try to make me buy them again, but is does not do anything at all when I click on the account button in the store, but the support articles that say to go to the apple store online and in your account to find downloadable purchases were no help. In fact although they are in my purchase history still, there is NOTHING about downloadable purchases. I called support about 2 weeks ago and they helped me about as much as a hole in the head. I can download the learn to play lessons that i've used with no problems, but the artist lessons are somehow nowhere to be found.
I am not about to try to purchase another artist lesson before this is fixed either.