Serendipity Labyrinth (in Bonus) appears unlocked when the Bonus category is selected. When the level is selected, the level soon grays out and in now unplayable. (It requires that the UT of The Spoils of Serendipity Gardens be beaten.) However, when I click next after playing Puzzle 8, the level is magically loadable, and can be played.
It seems the unlock requirements for the level could use some more work.
I have had this same bug, although it wasn't unlocked for me after Puzzle 8 for me. I'm not sure how it did happen for me, as I unknowingly played the level when it unlocked and then realised 'wait, this shouldn't be unlocked yet' - I didn't have the unlock requirement. This ought to be fixed.