It's probably a little too late now, but have you tried disabling "Attention Aware Features" in Face ID and Passcode Settings? iPhone will lower the volume of ringtones, alarms, and alerts when it thinks that someone is looking at the screen. (Emphasis on "thinks" - it easily gets fooled.)
The fact that it worked on the iPhone 7 (which doesn't have Face ID) suggests that this could be the issue.
Yea, I swear I made all possible steps and spend days trying to figure this out.
Since January
- I made 5 factory resets, 3 from Apple
- turned attention aware feature off
- turned face ID and passcode off
- uninstalled all apps that could be uninstalled
- exchanged 2 iPhones
- created new Apple ID (started from the scratch)
I was looking for a culprit with Apple support for 4 months without success. I told them from the beggining that's a global issue. Just search "iPhone ringtone cutting off" and you see tons of threads from tons of people experiencing the same issue. But Apple support isn't Apple itself.
I replicated the issue on 4 (!!) different devices. You can replicate this too. Here is how:
- open YouTube, play video, stop video and return to home screen
- open Music, play music, stop music and return to home screen
- open (Netflix, HBO MAX, Disney+, Apple TV... whatever streaming platform you have) and play video, stop video, return to home screen
- open Instagram (play stories/video) stop video and return to home screen
- open Twitter, play video, stop video, and return to home screen
And now... do all these steps once again. Once you're done, turn off display and call yourself a few times. Your phone will not ring or it will ring for 2 seconds, then it will go fully silent.
This is hard way and 100% accurate how to replicate the issue. In my case YouTube and YouTube Music was enough to stop my phone fron ringing.
I spend days trying to find out which app is preventing my phone to ring and then after factory reset I noticed you don't need to install any apps. You can replicate this issue just via Safari and Apple TV and Apple Music. Play and stop music a few times and your iPhone will stop ringing.
And another thing. This issue occured on iPhone 12 pro first. All models (13 and 14 are "infected") I have iPhone 7 and everything works fine there.
Btw, if you put your phone face down, there is much bigger chance your phone will not ring.