I'm using the application dashboard as an application launcher. (BTW, there is no component selectable for this one, since it is related too multi-screen, I selected that instead.)

With Plasma 5.4, I could place it in the panel and then assign a hotkey.
When the hotkey is pressed, the application dashboard would show up on the currently active screen (which for me depends on the mouse position).

With Plasma 5.5, this is no longer valid. The application dashboard always shows up on the screen which holds the widget.
In fact, since the dashboard in contrast to the other two launchers is not bound to a specific position on the screen, but instead occupies the complete screen, I don't think that such a restriction is necessary.

Instead, I would propose that:
1. There is no necessity to require a placed widget (in the panel or on the desktop) to activate the dashboard at all. It would make sense to allow to start the application dashboard solely based on hotkeys or (which I would very much like as well) triggered by an edge event. Or another trigger like a gesture (which could be linked to a hotkey, though).
2. The application dashboard should always show up on the currently active screen, since that is what the user currently focussed on and most likely wants to start the program there.

Reproducible: Always