Something about the way I worded this question helped me figure out the answer.
The Author's notifications disappear ... as you would expect
This made me think it might be a user permission problem rather than anything to do with the workflow step.
I confirmed this by switching the roles of the two user types, (so the Approver starts the workflow and the Author must approve the change). As I guessed, it was any notification assigned to the Approver, rather than notifications generated by that particular workflow step, which were not disappearing.
Specifically, the permission that was missing was for /etc/workflow/instances
. I allowed all methods for that user, and my problem is resolved.
You can edit permissions via /useradmin.html in your CQ5 instance (in my case, http://localhost:4502/useradmin.html
)