FWIW, I don't use Lux. Right now, the alarm dismiss now option, which had been mostly available recently, has been missing for more than a week. My working speculation is that its disappearance and reappearance has something to do with automatic updates to the core Google apps that came with my stock Nexus 5. I haven't kept track, but the change in alarm behavior seems to follow such updates. I'll note again that the same behavior always occurs simultaneously on BOTH my Nexus 5 phone and my 2012 Nexus 7 tablet. Both use Android 4.4.2. My alarms are set the same on both devices: daily alarms with the same label, five minutes apart. So ... whatever happens is apparently not unique to apps unique to one device or the other. The common thread I can see is from Google-supplied updates, though it is possible, I suppose, that it could come from my corporate e-mail accounts, which are also identically installed on both devices.
Whatever, it is a HUGE nuisance, and I wish that someone from Google would take note of this problem and help track down the cause and resolve it. As it is, we can't depend on the alarm dismiss function working reliably.