It appears the behavior is different between different client app.
- the Mac app 5.1 shows all channel members correctly
- the Mac app 5.2.2 shows no channel members
- the Windows app 5.2.2 shows no channel members
- the iOS app v2.0.0 shows no channel members
- the browser app shows no channel members
see screenshot below, ew have 10 members in the channel (as indicated by the headcount displayed under the group title on the top), but only one user is shown as channel member.
