uncollapsed fieldgroup_titles are invisible

Problem:
- Some filedgroup_titles (which act as trigger for collapsible behaviour) are invisible and therefore uncollapsible.

Pages tested (affected?):
- CCK node edit pages (yes)
- Modules - /admin/build/modules (yes)

Themes tested (affected?):
- Garland 5.1 (yes)
- Minelli 5.1 (yes)
- BlueMarine 5.1 (no)
- Pushbutton 5.1 (no)
- Chameleon 5.1 (no)
- Marvin 5.1 (no)

Browsers tested (affected?):
- Firefox version 2.0.0.2 (yes)

Odd behaviour:
- on page load the plain text title (eg core - optional) appears briefly but disappears immediately.
- on page unload ( ie after clicking a menu link to another page) the formatted link title appears briefly, just before the new page loads

Clues:
- The only fieldgroup_title that appears on /admin/build/modules is the 'core - required'. This is the only fieldgroup that is collapsed by default.
- By setting a CCK fieldgroup to 'collapsible:yes collapsed: no' - no title is displayed
- By setting a CCK fieldgroup to 'collapsible: no collapsed: no' - plain text title is displayed
- By setting a CCK fieldgroup to 'collapsible:yes collapsed:yes' - full functionality is achieved.