Alarm volume changes on its own
So, I've had this problem for a while as well. What I came to realize was that my sleepy-brain would hit the volume rocker to adjust the volume of the alarm while it was going off. Usually, this was to adjust the volume of my music to a more bearable level to wake up to once my phone was in-hand. But this also had the side-effect of adjusting my alarm volume to a point where next time it goes off, it's way too quiet.

I fixed this problem by downloading an app called MacroDroid. I set up a macro that executes each night at 10 pm. The macro starts by setting the alarm volume to 80%, pops a notification up that says "Don't forget to set your alarm!" And when the notification is clicked, it takes me to the Clock app to set my alarm. 

It's a bit of a workaround since it uses a third-party app, but it's been a huge quality-of-life adjustment for me so I figured I'd share.