It appears that a TagList widget in relation mode is not saving an empty value, because it no longer sends a POST value when empty.

Replication steps

In the Test plugin:

• Go to Users, edit either user.
• Click Tag List Field in the top alternative views box.
• Empty the Roles field, then try and save.

It does not fail validation, so the form saves, as the last value in the tag list is kept on refresh even though all tags are removed.