Thank you to whoever suggested to put the reminder at bugs.kde.org that we should probably post here first when not sure if it really is a bug or even a valid feature request.
This answered my question, because I have Picture of the Day on my secondary screen and tried “Accent color from wallpaper” today.
I was going to submit a minor bug for the fact that accent colour isn’t set from the wallpaper when different screens have different wallpapers, but after reading this I’ll just classify it as “works as designed - and the technical underpinnings will perhaps not support it anyway”.