Invisible attendees/( only visible for one )

we just had a conference with 9 attendees. One of the attendee (lets call him X), was not visible for the others except of one. So all could see each other, but only one could see X. X could see all.
Any idea ? Is there a limitation I missed to recognize ?

Is this a custom server or on the official deployment?

on the official deployment

have you tried to replicate this? if so report it here