Can't assign case role

The contact reference when assigning a contact to a new case role doesn't work. There is a text input box with a grey "- select contact -", but it doesn't have any select2 feature. Clicking Continue does nothing, even if you enter a name. However, entering a contact ID does save it.

To replicate:

• manage a case
• open the Roles section
• click the pencil to assign someone to the role
• you'll see the regular text box

This is the same both on a client's upgraded sandbox and the demo.