Merge requests from public to private project disappear

Summary

Merge Requests from a public project disappear when the target project is set to private.

Steps to reproduce

• Fork a public repository
• Create a merge request from the forked public repository to the fork
• Observe that the fork lists the merge request as expected.
• Make the fork private.

Example Project

• Source: https://gitlab.com/bengt/cwa-documentation-public/
• Fork: https://gitlab.com/pfasdr/cwa-documentation-private/
• Merge Request: https://gitlab.com/pfasdr/cwa-documentation-private/-/merge_requests/1

What is the current bug behavior?

• The private fork now lists the merge request as closed.
• The error message in the merge request:

The source project of this merge request has been removed.

• The source project is still not removed, rendering this error message incorrect.

What is the expected correct behavior?

• The merge request still works just like when the fork was still public.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

This bug happens on GitLab.com

Results of GitLab application Check

This bug happens on GitLab.com

Possible fixes

I don't know.