Steps to reproduce

1. Create a custom field of type 'Checkboxes' and add it to default screen
2. Create an issue and select one of the options (to make the field show up on 'view issue' screen)
3. In 'view issue' screen click the custom field, select one or more options and save changes
4. Click the custom field again

As a result the first option seems to disappear. Here's a screenshot showing what it looks like: