Disable crossfire Radeon settings>Gaming>Global
In crossfire mode all monitors must be connected to the main GPU
You can use MST hub +DP to DVI adapters to connect all of your monitors to your main card and use crossfire mode
My apologies for not answering back in over a year. I do not find this information accurate, for all 6 monitors were working before.
Anyways this is an old post, so I can going to create a new ticket for the current version.