Group milestones don't appear in group-level scoped issue board list

Summary

Open group milestones are not able to be selected in the milestone scope on group-level scoped issue boards.

Steps to reproduce

1. Create a group on GitLab.com on a Bronze subscription or higher.
2. Create one or more group milestones within that group that are Open and are either set to the current date or some time in the future.
3. Edit the default issue board for the group and attempt to set one of the newly created milestones as the milestone that the default issue board is scoped to.
4. Observe that when the Edit button next to the milestone scope is clicked that a loading circle will spin endlessly. If you click away and then click Edit once more it will load but no milestones will be displayed.

Example Project

170049

What is the current bug behavior?

When editing the milestone scope of a group-level issue board all open milestones at the group-level are unable to be selected and do not appear in the list.

What is the expected correct behavior?

That group-level milestones can be set as the scoped milestone on group-level issue boards.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com: 13.4.0-pre 1548643bab5

ZD: https://gitlab.zendesk.com/agent/tickets/170049 (GitLab Internal)

Workarounds

No workaround seems to exist for this issue yet. Attempting to update the milestone scope for a group-level issue board via the API returns a 200, but no change is made. The following two API calls were used to get the issue board ID and then update the milestone scope on the group board from the example project section above.

curl --request GET \
  --url https://gitlab.com/api/v4/groups/9025397/boards/ \
  --header 'authorization: Bearer TOKEN'

curl --request PUT \
  --url 'https://gitlab.com/api/v4/groups/9025397/boards/1953697?milestone_id=2' \
  --header 'authorization: Bearer TOKEN'