I'm on a fully updated KDE Neon (Plasma 5.16.3), using Amarok for music and Vivaldi as a web browser.

When I have a video playing in the browser and music at the same time (which is actually often), I want the media keys on my keyboard to control Amarok. To that end, I've tried opening the Media Player widget and selecting Amarok from the drop-down list, but the moment the widget closes (it's in the notification area) the media keys go back to controlling the browser video.

I've also tried to remove browser integration from Plasma in order to prevent the above from happening, but somehow it's still working.
The extension is not installed in Vivaldi (at least it doesn't appear in Tools > Extensions) and I've even removed the plasma-browser-integration package from my system. Still, media keys are able to control browser videos.

How can I ensure that the keys will always control the application I choose (Amarok) or alternatively, disable browser integration?