The auto brightness is not a bug. I believe they (CM team) actually implemented a more sophisticated criteria for the auto adjustment settings.
You can try to play around with it at Settings/ CyanogenMod settings/ User interface/ Automatic backlight. The reason it makes the screen to dim is because the room is dark, under the settings you can also adjust screen dim level (I think should be in percentage, default is 20). Else you can disable the auto brightness.
For me, I used notification power widgets and put toggle brightness modes to Auto/Dim/25%/75%/100% so I can easily switch modes whenever I feel the screen is too bright (bedtime) or too dim.
Edit: Nightly build 32 is up, download at official mirror
http://mirror.teamdouche.net/get/legend/cm_legend_full-32.zip