I'd been wondering about this issue for months, now.
At first, I thought I made a mistake in calculating the numbers of pages I unlocked. Just now, I was pretty sure I only unlocked 8 pages of a comic of 5 coins each. However, the 50 coins I just bought misteriously vanished when I was about to unlock the 9th page.
Has anyone experienced this before? What was wrong? I felt cheated... 
