Understanding meteor-cluster with Openlog enabled

Hey guys,
I’ve just released my app yesterday on my production system and now considering some strange behavior. I’m using the following structure:

MongoDB Replicaset:

On all of these sets is running an instance of my Meteor app. At the moment I’m using meteor-cluster. For deploying, I’m using meteor-up and have enabled OPLOG, to not fall back to polling.

Now, this is what happens:
If I login to my app, I can chat with other registrated test users and see all of my chats in the dashboard. After closing my app and reopen it again, suddenly all of my chats are lost and I only see a few of my contacts. After reopen it again, chats are still lost, but I see a few more of my contacts.

So it seems to me like I’m having an issue with MongoDB / Oplog. Is it wrong, to run an instance of Meteor on all members of the replica set? I’ve checked also my replica set status (rs.status()) and all slaves are synced to the primary.