I have since found a solution for this. It was because the segment was based on a success event that had visibility set to "builders". When I switched it to "visible everywhere" the error disappeared. But I agree it looks like a bug, so I've submitted it to Customer Care.
This is definitely looking like a bug. The steps are outlined really well here - if you can reference this thread when creating a ticket with Customer Care, along with your login company, username, and workspace name, that should be enough information for the agent to troubleshoot.