Looking through the images in https://duckduckgo.com/?q=Mistmantle...se-d37bzkq.jpg, it really looks as if it was custom made for that one title.
There is even one book without it. Perhaps it was misplaced.
Another, the publisher decided to squash things a little.
The French versions just use it for Mistmantle and omit the flourish for Chronicles; the Russion one didn't bother.

Acorn