Alarm volume keeps setting itself to silent

Starting a few weeks ago, the alarm volume on my phone keeps setting itself to silent.

If I go to Settings --> Sound and Notification --> Volumes, the Alarm volume setting keeps on resetting itself to silent. I can manually move it back to full volume, and it will stay there for some time, but eventually (within 24 hours) get set back to zero. I haven't been able to figure out any particular action I take that causes it to reset, or how long it takes exactly.

I have an HTC M9, running Android 6.0 Not rooted or anything.