milestone issue api and group issue api return different results

Summary

Running Gitlab 12.4.2-ee on prem , the milestone API returns only a subset of issues that the group issue API returns when filtering by milestone name or looking in the GUI.

Steps to reproduce

URL to get the issues from the group milestone:

• https://gitlab/api/v4/groups/group_id/milestones/milestone_id/issues?per_page=100 ... returns 18 results.
• https://gitlab/api/v4/groups/group_id/issues?milestone=milestone%20title&per_page=100 ... returns 44 results.
• https://gitlab/groups/my_group/-/issues?milestone_title=milestone+title ... returns 44 results (in GUI)

Example Project

On premises, can't provide example project.

What is the current bug behavior?

The milestone API does not return all issues attached to the milestone.

What is the expected correct behavior?

The milestone API should return all issues attached to the milestone rather than needing to use the group issue API.

The milestone issue API documentation doesn't have any explanation for this or indicate that it is expected behavior.