Same here. I remember this same problem on iOS 16 or 15 at some point, and then it got fixed in an update.
Definitely annoying, and especially noticeable when locking/unlocking the screen or when sending a whatsapp message.
I've noticed this bug for over a year on my iPhone XS intermittently. A reboot always returned volumes to normal levels. I found this still happens occasionally on my latest iPhone 14 Pro On iOS 17.0.3 too! 👀🤷♂️