There might be a bug too (or the button be switched by mistake).
It’s kind of a bug/feature/limitation thing (at the moment, given the internal structure of the code, we can show the preview of just one layer at a given time), in the future we’ll try to improve this.
I just noticed that when I switch between Selected and All, the change is made in all other dialog boxes with this button as well. That’s probably where the change took place.
I wonder if having an option in the settings to choose whether All or Selected is highlighted by default in all dialog boxes would help to mitigate some of the issue for users like myself (unless this already exists and I just missed it entirely).
Regardless, you’ve been incredibly helpful @dacap, thank you.