Cannot fork project into the same namespace

Summary

I created a group and than created a project within. I would like to fork this project into another project within the same namespace. The original group is missing from the namespace drop down list.

Steps to reproduce

  1. Create a group
  2. Create a project within that group
  3. Click "Fork" on the project main page
  4. Open the 'Select a namespace' drop down list and select the group name - you cannot, because it is not there.

Example Project

https://gitlab.com/triversum/php-skeleton

What is the current bug behavior?

There is no item in the 'Select a namespace' combo box with the name of the group.

What is the expected correct behavior?

There should be a namespace shown in the 'Select a namespace' combo box with the name of the group and I should be able to fork it. Should this be an unsupported functionality or should this be protected by some kind of setting or permission or it is behind the pay wall, than I'd like to ask this to be clearly and explicitly stated in the documentation.

There is also a disabled checkbox between the team settings called 'Prevent forking outside of the group' which I cannot find any documentation for but the very existence of it suggest that this should work.

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

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Results of GitLab application Check

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Possible fixes