TLDR: It’s an Android 6.0.1 bug that happens sometimes near the end of the month. The option should return tomorrow.
I can’t really believe it, but this seems to be a known bug in Marshmallow. I remembered that this happened on my Sony a year ago and found this in a forum
“Do not disturb until next alarm” option disappears every last week of every month, reappears 1st of every month. This is Google’s problem AFAIK, it’s there since Android Lollipop.
More can be read an Android Police, including this work-around:
After a little feedback from our commenters, we’ve found that 1) […] 2) it’s possible to work around it. If you turn off the automatic date function in the Android settings menu and return the date to something either before December 25th or after January 1st, the Until Next Alarm option returns.
It might even be related to holidays? Nowbody nows what Google did here, i can only confirm this is not happening on my Android 7.1.1 devices, neither the Pixel C tablet, nor the Xperia Z5 compact.
The only good thing is that the option it will probably return tomorrow. I hope Fairphone might backport the fix from Android 7? Since there is little chance for an official Android 7 release for the FP2?
@Douwe I think this is something for your FAQ/support team to know about.