In the speaker test page (the one with the buttons "front left" and "front right") the sound output always goes to the standard audio output device instead of the audio hardware the has been chosen by the dropdown "sound card". If I change the standard audio output on the "Volume" screen by setting the radio button to another sound card, go back to the speaker test page, then test sounds "front left/right" end up at the newly chosen device. I'm quite sure this worked in previous KDE versions, but can't say when it became broken.
If you set the default device through plasma-pa that messes with the dropdown I am almost certain.
Cannot confirm on Arch Linux, plasma 5.12.3.