Maintainers can select private group in MR Approvals but the group doesn't persist

Summary

Reported by Premium with 170 seats: https://gitlab.zendesk.com/agent/tickets/140158(internal only)

If a user is added as a maintainer in group/subgroup/project and they are not added to either group or subgroup, they are still able to search the group(or subgroup) by name and add it to the approvals from the project's settings, however, the members in the group are returned as none in both the MR approval rule's page and the MR.

Steps to reproduce

• Create project under group_name/subgroup
• Add a user(that is not a member of the group or subgroup) as a maintainer
• As the added user, navigate to the project's settings to update the MR approvals
• Type in the name of the group_name/subgroup
• Click Add
• Notice the members are listed as none

What is the current bug behavior?

Whether the maintainer should be able to add the group as approvers should be consistent with whether or not they can view the group in the list in the first place

What is the expected correct behavior?

Maintainers can search the group name and click Add, but then group doesn't persist

Relevant logs and/or screenshots