Use field default doesn't work

I have plain text, filtered HTML & full HTML formats.

Admins are allowed to use all formats and everyone else only gets plain text.

I have a text area field where the default test format is plain text.

When an admin user goes to the node edit page the default format they get is filtered HTML.

This is because on the text formats list page, plain text is heavier than filtered HTML.
If I change the order so plain text is lighter then the deafult becomes plain text.
This signifys that the 'Use field default' setting is not doing anything.

To reproduce:
* Have filtered html & plain text formats.
* Administrator role has access to both formats
* Filtered html has a lighter weight (higher in the list) than plain text on the format list page.
* 'Use field default' is checked.
* A text area field with filtered text has its field default set to plain text.
* All better formats permissions are unchecked for all roles.
* Go to the node edit page with an administrator user and the default will be filtered html, not plain text.