Group members are not displayed in project members page

Summary

We recently upgraded gitlab from 8.4 to 8.13.1 and we noticed a bug/feature when displaying project members.

Steps to reproduce

Create a project in a group, where the group containers a few users that have Developer access.

Add a few, not but all users in that group to the project member list. Add a few users that not in the group to the project member list.

Open project members page

Expected behavior

Both users in project's member list and in the group should be shown. As in the page below "Members" title it says "Existing users and groups".

Actual behavior

Only users in project's member list are shown. Other users in the group, are not displayed.