Are you sure it’s not actually auto-booting? There is an issue some of us are experiencing where there is a long delay prior to boot. (In my case, it is always 22 seconds before the boot logo appears whether I boot by opening the lid or by pressing the power button.)
I initially thought my MBP was not auto-booting, but there is just a long delay. It still hasn’t been fixed in beta 5. I did a clean install, too.