Ringtone/notifications randomly ignore volume settings and get incredibly loud

My ringtone/notifications randomly decide to ignore my volume settings and blast out at full volume; this includes the sounds of ringing, incoming texts, sending texts, locking my phone, plugging in my phone, app notifications, etc. It does not include music, podcasts/audiobooks, or videos.

The only way to get them to go back to what I have configured them to be is to restart my phone completely.

I keep my volume pretty low because sudden loud noises can trigger panic attacks and flashbacks pretty easily, but I can't leave my phone on silent 24/7 because I need to be reachable. Today, this issue literally almost resulted in serious injury or worse, so I'm can't tolerate it anymore. I'm hoping someone has experienced and fixed this issue, because I'm all out of ideas and I don't want to have to buy and learn how to use an Android :/

Phone model: iPhone SE

Note: this problem also happened on my iPhone 8 (and is the reason I bought a new phone last year), a family member's iPhone 14 and the 12 she had previously, and on several friends' iPhones (not sure which models they have. I think they're all at least 12s)

Version: 18.4.1 (up to date as of writing)

This problem predates the current version by at least 2-3 years. Nobody has been able to fix it or even determine the cause, even when it happens while they're looking at my phone.

Things I've noticed:

1. It can (sort of) be triggered by repeatedly unlocking and relocking my phone. The "click" that sounds when locking will start at the correct volume, then get progressively louder with each repetition, though it doesn't get as deafeningly loud as it does when this randomly happens.
2. It's very much random. It happens when I'm actively using my phone, and it happens when I haven't touched my phone for hours. It happens when I've been listening to music, and it happens when I have not. It happens when it is or has been recently connected to my car's Bluetooth, and it happens when it hasn't been connected for days.
3. When this issue happens, all of the volume controls/bars still show that the volume should be very low. So the problem is not that I'm somehow accidentally turning up the volume.

Things I've tried to fix it (based on searching for other people who have had this issue):

1. Going to the "Sounds & Haptics" settings and turning off "Change with buttons." This has made no difference at all.
2. Turning down the "Ringtone and alerts" volume slider. The slider is always at ~15% volume, even when my phone decides to blare.
3. People usually advise turning off the "Attention Aware" feature, but SEs do not have that feature.
4. An Apple tech once suggested that I turn on the "Limit Maximum Volume" feature, but that feature only affects media audio, not notifications/ringtones. I tried it just in case, and (as expected) it made no difference.
5. The same Apple tech ran a diagnostic on my phone and said everything looked good, and that he had no idea what was happening.
6. People often suggest I'm accidentally pressing the volume controls on the side of my phone when I put it to sleep, but "Change with buttons" is off. Just in case that setting wasn't actually working, I have tested and confirmed that that is not the issue.