Release can't be assigned to closed milestone

Summary

Since the feature has been released to assign milestones to releases, I wanted to do some housekeeping on past releases to add the milestones where appropriate. However, it seems closed milestones can't be added to releases, at least not in the GUI. These fields don't show them, the search doesn't show them, there's no way to select them.

Steps to reproduce

See above.

Example Project

Internal. Happy to be contacted by GitLab support if needed.

What is the current bug behavior?

See above.

What is the expected correct behavior?

It should be possible to assign closed milestones to a Release. They should probably not appear in the drop-down by default, but if you search for them or put a valid milestone link manually, it should be possible to assign them.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

Possible fixes