The Keep internal option is not shown in our Release info tab. The project has the same settings as other projects where the Keep internal is shown. Also Keep internal is shown in Issues-->Layout--> Description field
Thanks for reaching out to the community, and I would love to take a look at this for you, but I am not entirely sure what feature you are talking about, and hoping that you can provide some additional information and screenshots if possible to narrow this down, noting this is a public forum so make sure to obfuscate any information that you do not want visible to the public.
As you noted that you have "Keep internal" listed as a field under the field configuration as a description Field, Indicating this is a team-managed project, and Keep internal is a custom field for the project. However, I am not sure how you are tying this to the content in the release information unless you are trying to add in the field as a column bit it is currently not possible to customize columns to the release page as covered in the feature request:
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