Hello wukakuki,
There is a few things that could be causing this issue. Please check to see if your Intel Card is switching off and the Nvidia Card is running.
1.) Can you test this on 10.10?
If the card is working as intended and it does still occur in 10.10 then this could possibly be an engine issue. If lighting builds in 10.10 and not 10.11 then this could be a beta specific issue with Capitan.
Please let me know the answers to these questions and I will continue to investigate this issue in the meantime.