This just happened today! and I got my ass chewed.
My alarm was set for 0515 this morning just like it is every six weeks when I am on this 0630 - 1530 shift. Today I woke up a around 0400, looked at my phone and went back to sleep. I even checked my google hangout, so the phone was completely woken up. Went back to sleep and well, I was late.
I have hated this phone pretty much a few months after I got it, and here is just one more reason. I am going to clear the cache and use a third party app to prevent this until I can get a new device. If I remember I will come back with an update here next week with how those steps work out.
Glad I'm not the only one with this problem, though it should not be an issue.
Tried that, after a few days the third party alarm failed.