I have an issue, tottaly software problem i think. My volume buttons stops working, when i reboot the phone, then it works again, but temporarily, than i have to reboot again.
I'm running in to this on the mi Max 3 as well, not sure when it started, sometime in the 8.11.x round of updates, volume does change in sound settings, so if for whatever reason you're not in a position to reboot your phone you can do that.