I posted a reply earlier which was deleted due to it being "speculative", so here's the non-speculative facts. The problem lies in Headphone Notifications, a hearing safety feature implemented in 14.2 update.
Settings>Haptics and Sound>Headphone Safety is where the option to turn this off would normally be. Unfortunately, Apple has decided to not allow users from certain regions to turn it off.
Apple is citing "regulations and safety standards" of these regions as the reason, but that isn't a plausible explanation because there are no such regulations in my region and Apple is the only company implementing anything like this.
The "Reduce loud sounds" option is a different thing and doesn't prevent the notifications from being shown and lowering volume to 50%.
To put it simply, there is nothing we can do about it. And I'm very frustrated by this, because this feature is unspeculatively interfering with normal function of the phone. Not only do different headphones attain different loudness at the same volume level, but higher volume is necessary for speakers to be heard in a room.