'Required field' validation does not prevent node from being saved when edited

I’m using the most recent 7.x dev version. I have a term reference field that is required, with “force selection of deepest level” disabled and unlimited values allowed. If I’m creating new content, then the error message fires, but if I edit content and attempt to delete my selections (by choosing “-Select a value-“ for the parent) then I do not get an error message. Instead of validating, the node saves and the term reference field is populated with the first tid in the vocabulary. Should the validation prevent the node from being saved?