Just noticed this on my iPhone XS as well today, on iOS 13.5.1. While the lock sound was the most obvious one, it also affected the timer sound from my short time of testing it out -- but only when the timer reached zero while I was outside the clock app itself.
I almost always keep my phone on silent mode, but this is frustrating none the less.
I can also report the same of how the volume slider when using the physical volume buttons does not match nor change the volume slider in the control center. The two are now seemingly entirely disconnected from one another, and changing one does not change the other.
I did not even realize that was possible.
Lowering the volume through the control center seems to fix it, and although both sliders are still disconnected from one another I now have both set to 5-10%, so hopefully that should do the trick.
Edit: TIL I realized that iOS disconnects the ringer volume from the system volume, and this seems to be what's causing the bug to happen? Basically, it's not a bug, it's a feature...? Why is there no option to connect them to the same single volume slider? Why does the physical buttons either have to change the ringer volume (leaving the system volume untouched) or the system volume (leaving the ringer volume untouched). Why not have it change both?
Well, I am actually thoroughly baffled that I never realized this behavior until today, despite having used iPhones exclusively since the 3G model.
And I really don't like that design decision.
Anyway, so, solution is rather simple: Set the ringer volume (Settings > Sounds & Haptics > Ringer and Alerts volume slider) to the same as the system volume (Control Center > volume slider).
This screenshot highlights the difference in the volume sliders: