I discovered this same bug/malfunction, but this thread is short, so it seems pretty rare. I have an iPhone 6+ running 8.2. My phone is set to require a passcode, "immediately."
I was asking Siri about something and she brought up the safari list. I tapped on one and she opened the website. At that moment, I realized that she didn't ask me to unlock my phone first, like she does so often when I'm driving. It was immediately obvious to me that I could have unlocked Siri with the touch id, simply by activating Siri by holding the home button. What was strange is that Siri activated much more quickly than the time it usually takes for the touch id to unlock and bring me to my home screen. Basically, Siri's activation and unlock was faster than the normal unlock process.
With this strange occurrence, I began testing to see if Siri is using the touch id to unlock during the Siri activation process. I locked my phone and tested it several times, re-locking the phone using the lock button and illuminating the screen using the lock button. then using my usual finger, and asking Siri a question, to she if she would ask for an unlock. After 7 consecutive Siri activations, each time she brought me to the safari list, then to Safari, and to the home screen when I pushed the home button.
If Siri does use the touch id when being activated, then this is the sort of behavior I would expect.
Next I moved on to activating Siri by using my fingernail, from the lock screen, with no portion of skin touching the home button. Happily after tapping something on the safari list, it gave me a password prompt.
I relocked my phone and tried again. Unhappily, no password prompt was displayed through the entire process. Once I tapped on the safari list, safari opened right up. I used my fingernail to push the home button, and it took me home with no password prompt. I tried one more time, and again, no password prompt, all the way to home.
Extremely disappointed, but not really surprised, since Apple has had issues with Siri unlocking phones for years now. After those 2 failures out of three, I immediately turned siri access from the lock screen, off, and that was that.
For my use, I don't access Siri without my finger being on the home button anyway, such as asking Siri things via bluetooth or by saying "hey siri." I figured that I don't need to access Siri from the lock screen anyway, since my phone should quickly unlock while I'm holding my finger on the home button, right?
What's strange is that over the last few days, when I push and hold the home button, my screen illuminates, but does not unlock until I release the home button. Siri does not activate, no matter how long I hold the home button to unlock it. Now to get siri to work, I have to activate her from an unlocked home screen, since the touch id will not unlock my phone unless the home button is not being pressed.
Because of this irritation, I started looking online to see how serious this siri-unlocking issue is, this time around, and to see if there were any workarounds. To my surprise, it seems quite rare, which is unfortunate, since that means it will probably take years before it is fixed, and for those years, I'll have to keep siri out of my lock screen, and I'll have to do this two-step process to use Siri.