I really don’t like it being under 100%, especially when it doesn’t inform me about it when it happens.
This is specifically how Youtube volume works. If you change the volume on the player, you’ll notice it changes the app (or more accurately the tab) volume.
Youtube also applies loudness normalization so 100% on the player is not always truly 100%, hence how you probably noticed it being changed.
I don’t know if we can do things about it.
No KDE code is doing this; it’s one or more of the apps you’re using.
I’m not aware of a way to globally disable it, unfortunately. Most of the apps that do this (booooo!) include a way to turn off the behavior, though. Usually.

