Never faced anything thelike ... are you sure it's not a power button hardware issue ?
If you place a power button on the navbar, does that one work as expected ?
Sent from my Nexus 6 running Yank555.lu SlimLP 5.1.1 on Yank555.lu v1.3a kernel.
i have had the same problem with the 0.8 20150925-2115 rom, power button did not lock the phone... But a reboot fixed it!
I'm not having this problem on 0.8 20150926-1141....