This is apparently due to the fact that the stash commit is represented as a merge (between its parent and indexing state at that time), and format-patchdoes nothing in the merge transaction.
If you say
git format-patch stash@{0}{,^}
then he will spit out patches between the cache and each parent.
To illustrate, it looks like:
* 99aedb8 (refs/stash) WIP on master: 668ff36 initial commit
|\
| * 6b8d77f index on master: 668ff36 initial commit
|/
* 668ff36 (HEAD, master) initial commit
source
share