I hadn’t seen this done before. I hope that the practice doesn’t catch on. 
If you purchased the album in July 2009 and downloaded the four tracks that were available then, and then later downloaded track 5 after it was released in August, are your copies of the first four tracks now part of the five-track release instead? That’s pretty strange from a tagging perspective.
With physical media, it seems like it usually takes far more effort to actually create and distribute a new release than it does to add it to MB.
With digital (online) media, “releasing” something can be easier than adding it to MB.
(Algorithmically-generated content seems like it’s going to push things much further in this direction, but discussion of that that probably belongs in the “Implications of AI generated content” thread).