"404 Page not found" when clicking on a subgroup's milestone in the parent group milestones view.

Summary

"404 Page not found" when clicking on a subgroup's milestone in the parent group milestones view.

Steps to reproduce

1. Create a Top-level group
2. Create a subgroup
3. Create a milestone for the subgroup
4. Navigate to the top-level group milestones screen
5. Click on the milestone from the subgroup
6. Instead of the milestone, you get 404 Not Found page.

Example Project

Tested Project : https://gitlab.com/groups/test_group19/-/milestones

The issue can be reproduced on gitlab.com and we observe it on our on-prem docker instance v12.9 (and 12.8.x)

What is the current bug behavior?

Page not found

What is the expected correct behavior?

Open the subgroup milestone

Relevant logs and/or screenshots

subgroup_milestone_404error

screenshots