The conference server was not carrying SIP subscriptions for chatroom participants on server starting. This was caused by linphone::CoreListener::onParticipantRegistrationSubscriptionRequested() callback, which was called by the Core before it is fully started.
That especially was causing problems when the subrcription was implemented by SIP subscribe/notify mechanism, which is so when the target of the subscription is a user of a tier domain. As the Core was not fully started and the conference server needs it to send SIP requests, no subscription for tier domain users was done on starting.
Fix done in Linphone SDK.