Subgroup and project milestones aren't shown on group roadmap

Summary

While viewing the Roadmap page for a group, users are unable to see milestones created at the subgroup or project level. Only milestones created at the group level are shown.

Steps to reproduce

1. Create the following structure within a group on GitLab.com that has at least a Silver subscription applied to it:

  graph LR;
  A[Group]-->B[Subgroup];
  B-->C[Project];

2. Create the following:

• Group level milestone
• Subgroup level milestone
• Project level milestone

3. Set a start date and end date for all three milestones.

4. Navigate to the roadmap section of the top-level group.

5. Observe that only the group level milestone appears on the roadmap.

Example Project

• Group - Milestone
  • Roadmap
• Subgroup - Milestone
• Project - Milestone

What is the current bug behavior?

Only group level milestones appears on the roadmap.

What is the expected correct behavior?

Group level, subgroup level, and project level milestones should all display on the roadmap.

Output of checks

This bug happens on GitLab.com: 12.10.0-pre b7c8d2dbb8f

ZD: https://gitlab.zendesk.com/agent/tickets/153431 (GitLab Internal)

Design

Figma: https://www.figma.com/file/nEx7pjtBauZY621F4G6m91/214375?node-id=0%3A1