This all springs from the revamped notification scheme that started in Android 5.0 Lollipop, which eschewed the traditional way of muting a phone — by hitting volume-down until you reach the "silent" option. Instead, you had to choose from receiving all notifications, priority notifications (which included alarms), or no notifications. Android 5.1 introduced "Do not disturb until next alarm," which essentially kept you from missing alarms in this cockamamie scheme. It disappeared in Android 6.0 Marshmallow — which otherwise brought some sanity to this whole notifications thing —but returned in early December with Android 6.0.1.
And now it seems to have disappeared again.
We don't every single one of our Marshmallow devices in front of us at the moment (this being a holiday weekend and all), but it definitely doesn't appear to be localized to the Nexus 6P, which is where the reports of the disappearance started to trickle in. Reddit seems to think there was a date-based kill switch here, which certainly is plausible.
There's still an option for allowing alarms through for a set amount of time, but that's not the same as allowing an alarm to fire and also kill DND.