There is definitely something odd going on with the sound level volume being low for certain types of notifications--specifically any of the non-Ringtone alerts, e.g., Text Tone, New Mail, etc.; hear me out--on my iPhone 14 Pro Max (iOS 17.3.1--however, issue has been present in numerous previous iOS versions) I can replicate the notification volume level issue as follows: I have the "Sleep Time Free" app installed on my phone; when I enable/start the sleep timer on it, then go into the phone Settings -> Sounds & Haptics -> Text Tone and select any one of my alert tones I notice that the alert tone volume is very loud...but when I stop the sleep timer in the Sleep Time Free app and then go back into Settings -> Sounds & Haptics -> Text Tone and select the (same) alert tone, the alert tone volume is now noticeably quite a bit lower; I can replicate this at will. It is not just one specific alert tone either--I can select a different alert tone (even the iOS-provided ones) and it'll do the same high/low volume thing too, and as previously mentioned it occurs on any of the other non-Ringtone alert types too; oddly enough the Ringtone volume stays at a consistent volume level regardless of whether I start the sleep timer in the app. And actually you don't really even need to go into the Sound settings and select an alert tone to perform the test--the phone lock sound is affected similarly as well.
Obviously I'm not gonna be letting the sleep timer run 24x7 just so that my text and/or other notification alert tones will sound very loud but clearly someway, somehow the Sleep Time Free app is able to manipulate the notification volume level. Kinda odd that starting the sleep timer (in the app) would actually cause the alert tone volume to increase--you'd think it should have it decreased so that you're not awakened by a very loud alert tone notification while you're asleep.