Apple iPhone 12 alarm volume is going high and then going down. Volume does not stay high for a while. What can be done to maintain volume?

How to change the alarm's volume

1. Go to Settings > Sounds & Haptics.
2. Under Ringer and Alerts, drag the slider left or right to set the volume. As you drag, an alert will play, so you can hear how the volume changes.
3. Turn on Change with Buttons to use the volume buttons on your device to change the alarm volume.

How to set and change alarms on your iPhone - Apple Support

Lock the ringer and alert volumes in Settings

1. Go to Settings .
2. Tap Sounds & Haptics (on supported models) or Sounds (on other iPhone models).
3. Turn off Change with Buttons.

Try, "change with buttons" option, [Disabled]

I have tried changing the slider under "ringer and alerts" and turning on or off the "change with buttons" and my alarm continues to blare, then get almost silent. Why doesn't the sound volume work like normal when setting an alarm? I've seen many people having this issue and nothing I've tried from input has worked. Please advise!

Go to Settings/Accessibility/FaceID & Attention and read the description under Attention Aware Features. This may be what is causing the volume to change when you are looking at the screen.