Opening a subgroup milestone from the parent group milestones view returns 404

Summary

Opening a milestone from the parent group milestones view returns 404 Not Found page.

Steps to reproduce

• Create a top-level group
• Create a subgroup
• Create a milestone for the subgroup
• Navigate to the top-level group milestones screen
• Click on the milestone from the subgroup
• Instead of the milestone, you get 404 Not Found page.
• Go back to the top-level group milestones screen
• Click on Close milestone
• You get 404 again

Example Project

Here is the setup: https://gitlab.com/groups/milestone-issue/-/milestones

Follow the link and click on the test-subgroup-milestone. You get 404.

The issue can be reproduced on gitlab.com and we observe it on our on-prem Premium instance v12.8.

What is the current bug behavior?

Page not found.

What is the expected correct behavior?

Open the subgroup milestone.