Something to be aware of.

Custom fields are stored in JSON format along with the issue details. If an issue is edited which has an associated custom field group, when the issue is saved the 'custom field' entry is populated automatically. One consequence of this is that if the custom field group only contains 'fields' that have a 'default' value these will be automatically applied. This may not be immediately expected if all of the custom fields are 'optionally' supplied. If viewed in the front end then the 'custom field group' display may only show the fields with the 'defaults'. This is not a problem but a consequence of the design choice for the custom field group fields.