I have the same issue on my current Z4MC. It seemed to be related to the light sensor on the front of the LCM (light control module, aka the headlight switch), so I spent big bucks to replace it - and it didn't solve the problem.
My original Z4MC that i owned from new had a problem with the dimmer rheostat and the LCM was replaced under warranty. But those symptoms were very obvious when spinning the adjustment wheel. This is different.. just dimmer than it should be. Could it be the cluster itself? I dunno but I'm not swapping it out to try to find out. No codes being thrown at all. Would love to figure this out.