As soon as you drag any element inside the contents of a checkbox field, it is locked there forever. Attempting to delete it will delete the parent checkbox, and attempting to move it will move the parent checkbox.
@rowan@ctrav thanks again so much for posting this! I just wanted to follow up here and let you know that we pushed an update for this.
Going forward you will not be able to nest elements directly inside of the checkbox wrapper. Instead, if you need to add additional elements around the checkbox, you can place them next to the wrapper of checkboxes rather then directly inside.
This will prevent any element from getting stuck in the checkbox wrapper.