If a fieldset is on a 2nd level Field Group, dragging it sideways will remove it from the Field Group but will create additional Field Groups.
Steps to Reproduce
- Go to Content & Data > Web Content > Structures tab > Create new structure;
- Insert any field to the structure;
- Go to Fieldset tab and drag a created fieldset;
- Insert the Fieldset on a Field Group inside another Field Group;
- Drag the fieldset sideways (bug)
It should not be possible to create additional Field Groups by dragging the fieldset.
The fieldset is removed from the Field Group, but each time it's dragged sideways it will create another Field Group with only the first field.