To hear the iPhone X alarm at full volume lock the screen and place the phone out of view of your face.
I just got an iPhone X (am I a fool?) and it seems that the alarm volume drops when the phone is unlocked. If you use Face ID and you are looking at the screen then it is most likely unlocked and the alarm volume will be soft.
If the phone is locked when the alarm goes off it will be at full volume until you pick it up and look at it.