not working with checkboxes

I have a field group ‘ad’ with many fields including the ‘field type’ – ‘checkboxes’.
The rule being used is: Show this field group if post type = ‘accept/deny’
This field group is being used by a gravity form.
When the form is submitted a new custom post ‘accept/deny’ is created showing all the fields in the field group ‘ad’.

This is working as it should except for one type of field – the checkbox field. If a user checks more then one box on the gavity form and submits the form, only one of the boxes is showing as checked in the custom post that is created.

This looks like a bug to me. Everything is set up correctly.
Please can you look into this.

I’m using checkboxes in other forms in the same way and it is working fine.