My X is completely silent with the volume turned down... and as always been.
Tested on recorded videos I have made, tested on YouTube videos, tested on Facebook videos, tested on Spotify (not video) and Apple Music (not video) and again when the volume is turned right down, you get zero audio through the speakers.
What phones have you tried this on to give you the title; "iPhone X/XS/XSM: no mute?" because I'm almost certain you've not tested this on all those devices.
It may be a glitch with the phone your using, which model that is you don't specify, but if there's a problem try a software update/re-install or take it into an Apple store. If it's a new phone, take it into Apple and they will sort this issue out as it's not widespread at all, and we aren't all fanboy's on here for what it's worth.