I'm facing the same issue with Omar's Rom (I did not try the latest from the 16th of July).
I am not sure if you are using XPosed + modules - this is the case for me.
So if I dirty flash the rom and wipe Dalvik+Cache, reboot - the phone is alive and the power button seems to operate as expected. And I have not seen reboot because of that.
Now, if I enable the XPosed Framework then the troubles begin.... Made some trials and I have figure that, in my case, if I have the XPrivacy module active, then the problems appear: the power button would not really lock the phone. Instead the screen will get dimmed, only status bar visible and eventually the phone will reboot. The same happens if I do not switch off the screen with the power button but let the screen timeout kick in.