Alarm volume starts loud and gradually decreases by itself ..?

Open the Clock app, tap the Alarm tab, then tap Edit. Tap the alarm, then tap Sound and choose a Sound. If you connect headphones to your iPhone, the alarm will play at a set volume through the built-in speakers on your iPhone as well as wired and wireless headphones.

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

Does it do this when you look at the screen?

Turn off Attention Aware Features in Settings, Accessibility, Face ID & Attention

Great! Thanks for feeding back.