Title says it. After updating to Debian 8.4 I find that the power button no longer turns off the system. With older distros I could press power once and the unit would gracefully shut down completely. Now, I have to hold it for a period of time. The unit will eventually shut off, then immediately reboot again.
Am I missing something?
$ sudo apt-get install acpid
Give that a whirl, it should fix your issue. You changed images didnt you ? from LXDE image to one of the command line images. Right ?
If by chance that does not work for you. Use the latest image. Jessie 8.5 I believe. It works, or at least works for me, with the beaglebone black or beaglebone green.