Signup settings lost when a user edits the node without administer signup permission

If a user is able to edit a signup-enabled node but doesn't have the administer signup permissions then the signup settings for that node get lost (revert to default) when tht user saves the node. I'm seeing this with the owner of a group who has the ability to edit all the content in her group, but doesn't have administer signup permissions.

Note that signups themselves are preserved, it's just the settings that are lost.