This bug is reproduced inside plugins.

1) When an item for type begin_fieldset and end_fieldset is defined inside function get_widget_param_definitions( $params ) the fieldset Settings disapears.

2) If an item for type html is added, the fieldset Settings becomes visible, however the previous added fieldset is not nested correctly.

3) One would think it should look like this:

4) I am aware that it was probably designed to work like it is at the moment (Allow defined fieldset to 'override the default item'), but it does look weird if the Settings block (fieldset) is empty.

5) Proposed solution: https://github.com/b2evolution/b2evolution/pull/73/files